Particular bit of fish, unpleasant (7)
I believe the answer is:
finicky
'particular' is the definition.
(I've seen this before)
'bit of fish unpleasant' is the wordplay.
'bit of fish' becomes 'fin' ('fin' is part of 'fish').
'unpleasant' becomes 'icky' (icky can mean disgusting).
'fin'+'icky'='FINICKY'
(Other definitions for finicky that I've seen before include "Intricate; fussy" , "particular" .)