Agent suffering setback (and cut off) kept going (10)
I believe the answer is:
'kept going' is the definition.
(I've seen this before)
'agent suffering setback and cut off' is the wordplay.
'agent' becomes 'rep' (short for representative).
'suffering setback' says the letters should be written in reverse.
'and' is a charade indicator (letters next to each other).
'cut off' becomes 'severed' (synonyms).
'rep' in reverse letter order is 'per'.
(Other definitions for persevered that I've seen before include "Kept going" , "Was not deterred" , "didn't give up" .)