Press on agent after return, strict (9)
I believe the answer is:
persevere
'press on' is the definition.
(I've seen this before)
'agent after return strict' is the wordplay.
'agent' becomes 'rep' (short for representative).
'after return' says the letters should be written backwards.
'strict' becomes 'severe' (I've seen this in another clue).
'rep' in reverse letter order is 'per'.
'per'+'severe'='PERSEVERE'
(Other definitions for persevere that I've seen before include "Continue despite difficulties" , "Struggle on!" , "Serve peer (anag.)" , "hold on" , "Be persistent, refuse to stop" .)