Mercenary ruing he’d fired (5,3)
I believe the answer is:
hired gun
'mercenary' is the definition.
(I've seen this before)
'ruing he'd fired' is the wordplay.
'fired' indicates an anagram (I've seen this in other clues).
'ruing'+'hed'='ruinghed'
'ruinghed' with letters rearranged gives 'HIRED GUN'.
(Other definitions for hired gun that I've seen before include "can be mercenary" , "Professional killer" , "Hit man" , "Paid assassin" , "Professional hitman" .)