Spoil fish at home with last bit of cheese spread (9)
I believe the answer is:
margarine
'spread' is the definition.
(margarine is a spread)
'spoil fish at home with last bit of cheese' is the wordplay.
'spoil' becomes 'mar' (synonyms).
'fish' becomes 'gar' (type of gar).
'at home' becomes 'in' ('I'm in' can mean 'I'm at home').
'with' means one lot of letters go next to another.
'last bit of' suggests the final letters.
The final letter of 'cheese' is 'e'.
'mar'+'gar'+'in'+'e'='MARGARINE'
(Other definitions for margarine that I've seen before include "Vegetable fat" , "Substitute spread for butter" , "Reg and Marian make a butter substitute" , "F [FAT]" , "Is Ma angrier at this spread?" .)