It could be batwing, butterfly, leg-of-mutton or slit (6)
![Ross](/clue/static/ross-mug.min.png)
I believe the answer is:
sleeve
(Other definitions for sleeve that I've seen before include "It's attached to the armhole" , "Part of a shirt - old record holder" , "Part of clothing covering arm" , "An arm goes into it - or a music album" , "Possibly hiding-place for ace" .)