Upbraid fish salesman first (8)
I believe the answer is:
reproach
'upbraid' is the definition.
(I know that upbraid can be written as reproach)
'fish salesman first' is the wordplay.
'fish' becomes 'roach' (roach is a type of fish).
'salesman' becomes 'rep' (sales rep).
'first' means one lot of letters go next to another.
'roach' 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" .)