Bag holding the last bit of mutton for a bite to eat (5)
I believe the answer is:
snack
'bite to eat' is the definition.
(I've seen this before)
'bag holding the last bit of mutton' is the wordplay.
'bag' becomes 'sack' (both can mean a container).
'holding' means one lot of letters goes inside another.
'the last bit of' says to take the final letters.
The final letter of 'mutton' is 'n'.
'sack' placed around 'n' is 'SNACK'.
'for a' acts as a link.
(Other definitions for snack that I've seen before include "Small or huried meal" , "fast food" , "(Eat) a light meal" , "Quick bite to eat" , "Light bite" .)