Agent suffering setback? Then hard to keep going (9)
I believe the answer is:
persevere
'keep going' is the definition.
(I've seen this before)
'agent suffering setback? then hard' is the wordplay.
'agent' becomes 'rep' (short for representative).
'suffering setback?' says the letters should be written backwards.
'then' is a charade indicator (letters next to each other).
'hard' becomes 'severe' (synonyms).
'rep' backwards is 'per'.
'per'+'severe'='PERSEVERE'
'to' acts as a link.
(Other definitions for persevere that I've seen before include "Carry on regardless" , "Persist steadily" , "Persist, stick at" , "Be unwilling to stop" , "give it another try" .)