Retired agent is first of heroes to die (6)
I believe the answer is:
perish
'to die' is the definition.
('perish' can be a synonym of 'die')
'retired agent is first of heroes' is the wordplay.
'retired' says the letters should be written backwards.
'agent' becomes 'rep' (short for representative).
'first of heroes' becomes 'h' (1st letter of 'heroes').
'rep' backwards is 'per'.
'per'+'is'+'h'='PERISH'
(Other definitions for perish that I've seen before include "Die; decay" , "Go off" , "Wither or die" , "Deteriorate, rot" , "Expire" .)