Knot for fastening a leg of mutton? (10)
I believe the answer is:
sheepshank
'knot' is the definition.
(I know that sheepshank is a type of knot)
'fastening a leg of mutton?' is the wordplay.
I cannot really understand how this works, but
'a' could be 'an' and 'an' is present in the answer.
This may be the basis of the clue (or it may be nonsense).
'for' is the link.
(Other definitions for sheepshank that I've seen before include "Cut of meat" , "tied with this?" , "PS He has Ken (anag) -- rope-shortening knot" , "Knot used to shorten a rope temporarily" , "It's not without hitches" .)