It's definitely not easy, because kids don't want to do it. It's right in the mix with any other thing you tell them to do and they refuse: discipline. My 3 and 5 year old get told to clean up the toy room and their room daily.
I turn on music and say "OK, time to clean up the toy room. When you're done, we'll eat lunch-(or whatever we're doing after so they know there is a deadline.) You have until 11:30." My oldest tells time and I usually give a half hour even though I can whip through and have everything in it's place 5 minutes.
Then I give each child their first specific "job" "So and So, your job is to put the stuffed animals in their bin, and So and So, your job is to put all your cars back in their bucket. I'll be back in five minutes to make sure it's done and give you your next job."
Then in like, 30 seconds I yell, "Are you done? Should I come check?!!!" and they scream "Not yet mom!!!" and I keep them going by yelling every minute or so (it's our game). Then, at the deadline I say, "OK, here I come" and they scatter to finish up and get their next task.
Yes, sometimes they get distracted and start playing and need some counting and prodding, but they know if they do not proceed they will get swatted for disobeying if I make it to "3" and they're not doing their task.
If that didn't work we would compound it with toy removal in addition and stuff, but it's never come to that. They haven't needed any consequences in a long time because they know we're serious and they have never succeeded in refusing to do it. For their room, same thing. There is only a couple of toys, and a bin and dress up drawer and the day's clothes on the floor, so they have to cram the dress up clothes back in the drawer, the toys in the bin, and put their clothes in the hamper every time, and I come check in 10 minutes.
You need to break it down into smaller jobs. My kids wouldn't just "clean the whole toy room" either. I have to give step by step instructions, but my 5 year old is kicking in now and proceeding un-prodded and giving her brother jobs sometimes.
You need firmer consequences for refusing. She shouldn't succeed in losing 6 hours and still not having a clean room. Is this the ONLY time she refuses to do what you say? If so, what makes her mind you other times? Use discipline that works. If she's often not doing what you say, get firmer and be consistent for all things, so no matter what you tell her, she gets in the habit of listening.
And de-cluttering is essential. I do it all the time. We have never bought a toy in our lives, but somehow, from gifts, the house is packed to the rafters. I get rid of boxes of stuff monthly. They don't play anyway unless things are orderly. My friend lives in a tiny Manhattan apartment and ALL her kid's possessions are under his loft bed. Arranged neatly. It's enough, because for his space he has activities and they can be put in their place.
Firm up! Talking safety won't get through to a 5 year old and neither will reading a book form outside the mess! Make her do it! If you're not a spanker, maybe remove all of her stuff from her room that is out of place and give it away etc, but it's better to train her to proceed with cleaning as it's happening than to punish after the fact.