Rebuke salesman with fish (8)
I believe the answer is:
reproach
'rebuke' is the definition.
(reproach is a kind of rebuke)
'salesman with fish' is the wordplay.
'salesman' becomes 'rep' (sales rep).
'with' says to put letters next to each other.
'fish' becomes 'roach' (roach is a type of fish).
'rep'+'roach'='REPROACH'
(Other definitions for reproach that I've seen before include "Express disapproval to (someone)" , "Censure - or preach (anag)" , "Take to task" , "Chide" , "An expression of disapproval" .)