Tick off salesman over fish (8)
I believe the answer is:
reproach
'tick off' is the definition.
(thesaurus)
'salesman over fish' is the wordplay.
'salesman' becomes 'rep' (I've seen this before).
'over' means one lot of letters go next to another.
'fish' becomes 'roach' (roach is a kind of fish**).
'rep'+'roach'='REPROACH'
(Other definitions for reproach that I've seen before include "Reprove gently" , "approval? Just the opposite" , "Take to task" , "carpet" , "Berate" .)