Finished source of mutton before one serving of ham (8)
I believe the answer is:
pastrami
'serving of ham' is the definition.
(type of ham)
'finished source of mutton before one' is the wordplay.
'finished' becomes 'past' (I've seen this in another clue).
'source of mutton' becomes 'ram' (a ram's flesh might be used as mutton).
'before' means one lot of letters go next to another.
'one' becomes 'i' (Roman numeral).
'past'+'ram'+'i'='PASTRAMI'
(Other definitions for pastrami that I've seen before include "smoked joint" , "Tapered tuck; missile" , "Seasoned smoked beef, served in thin slices" , "Spiced smoked beef" , "Spiced sliced beef" .)