Knot for fastening a leg of mutton? (10)
I believe the answer is:
sheepshank
'knot' is the definition.
(sheepshank is a kind of knot)
'fastening a leg of mutton?' is the wordplay.
I cannot quite see how this works, but
'a' could be 'an' and 'an' is located in the answer.
This may be the basis of the clue (or it may be nonsense).
'for' acts as a 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" .)