Fish salesman's first criticism (8)
I believe the answer is:
reproach
'criticism' is the definition.
(reproach is a kind of criticism)
'fish salesman's first' is the wordplay.
'fish' becomes 'roach' (roach is a kind of fish**).
'salesman' becomes 'rep' (I've seen this before).
'first' is a charade indicator (letters next to each other).
'roach' put after 'rep' is 'REPROACH'.
(Other definitions for reproach that I've seen before include "Take to task" , "Express disapproval, admonish" , "Blame, scold" , "criticise" , "Castigate" .)