Upbraid fish salesman first (8)
I believe the answer is:
reproach
'upbraid' is the definition.
(synonyms)
'fish salesman first' is the wordplay.
'fish' becomes 'roach' (roach is a type of fish).
'salesman' becomes 'rep' (sales rep).
'first' says to put letters next to each other.
'roach' put after 'rep' is 'REPROACH'.
(Other definitions for reproach that I've seen before include "Blame, scold" , "Admonition" , "Express disapproval or disappointment" , "approval? Just the opposite" , "carpet" .)