Extremity one struggles to cut? (7)
I believe the answer is:
toenail
'cut?' is the definition.
Although both the answer and definition are singular nouns, I can't see how one could define the other.
'extremity one struggles' is the wordplay.
'extremity' becomes 'toe' (toe is a kind of extremity).
'one struggles' becomes 'nail' (I can't explain this - if you can you should give a lot more credence to this answer).
'toe'+'nail'='TOENAIL'
'to' is the link.
(Other definitions for toenail that I've seen before include "Part of body perhaps painted" , "One's occasionally filed" , "A girl might paint this" , "Digital cover-- 21 across (anag)" , "woman in labour" .)