Sit back to eat cold, whole shellfish (8)
I believe the answer is:
escallop
'shellfish' is the definition.
(I know that escallop is a type of shellfish)
'sit back to eat cold whole' is the wordplay.
'sit' becomes 'pose' (as in sitting for a painter).
'back' is a reversal indicator.
'to eat' is an insertion indicator.
'cold' becomes 'C' (eg on taps).
'whole' becomes 'all' (I have seen 'The whole ' mean 'all' so perhaps 'whole' could also mean 'all').
'pose' in reverse letter order is 'esop'.
'c'+'all'='call'
'esop' placed around 'call' is 'ESCALLOP'.
(Other definitions for escallop that I've seen before include "Heraldic shell; thin piece of meat" , "Thin slice of veal" , "Fan-shaped shell" , "Shellfish" , "Seafood" .)