Particular, distasteful part of fish to start with (7)
I believe the answer is:
finicky
'particular' is the definition.
(I've seen this before)
'distasteful part of fish to start with' is the wordplay.
'distasteful' becomes 'icky' (I've seen this in another clue).
'part of fish' becomes 'fin' (fish have fins).
'to start with' says to put letters next to each other.
'icky' put after 'fin' is 'FINICKY'.
(Other definitions for finicky that I've seen before include "Intricate; fussy" , "particular" .)