I stole a cat a while ago. Erm, that's maybe not a good way to start this story...
My ex-boyfriend's neighbour had 3 cats. Two were elderly - Rajah was 14ish, the older one was near 20, and the third was young and robust and male (other two were female). The man just put a bowl of food out every day and never let them in. The young cat would beat the older two away from the bowl, and their owner didn't care - so when I saw Rajah becoming worse and worse I decided that if he didn't do something by the time it got colder I would take her home with me and take her to a vet. She got steadily skinnier, and then one day after I'd not seen her for a week I saw her and she had stopped grooming. All of her fur hung in clots and snarls, and she'd lost even more weight.
So I went home and fetched my cat carrier (which I had despite not having a cat at the time - always prepared for one though as most of my cats have been impromptu rescues, though this is the only one I nicked) and we called her inside my ex's house, put her in the box and rang a taxi.
Straight to the vet's, and he said that if she'd been left outside for another week, she probably would've frozen to death
Anyway, my point is... (sorry, lost track there) ...I used to comb her fur out, one small bit at a time. Some of the bigger clots had to be cut out carefully and then had to grow back. It took me about a month to get her fur all sorted (but she had it messed up
all over), but it was worth it to do it slowly so she didn't get traumatised by an attempt to do it all at the same time.
If she won't allow you near it, have you checked to see if she has any injuries there, or maybe even a rash of some sort? It might be worth it to take her into a vet and have them check; they'll also be able to give you advice on how to remove it.
That said, I also had a cat who was too fat to wash his back (a neighbour overfed him all the time) and ended up having to hold him down and comb clots out of his back. I'm still surprised he never actually managed to claw me, he did
try!
Edit: I didn't steal the 20-year-old because she was being fed by other neighbours and was being cared for by them as well.