Admonish agent over insect (8)
I believe the answer is:
reproach
'admonish' is the definition.
(thesaurus)
'agent over insect' is the wordplay.
'agent' becomes 'rep' (short for representative).
'over' says to put letters next to each other (in a down clue, some letters go over others).
'insect' becomes 'roach' (roach is a kind of insect).
'rep'+'roach'='REPROACH'
(Other definitions for reproach that I've seen before include "Chide" , "Admonition" , "Express disapproval or disappointment" , "Castigate" , "Upbraiding, censure" .)