After pay increase, he will run riot (5,4)
I believe the answer is:
raise hell
'run riot' is the definition.
(both can mean to behave in an unrestrained way)
'after pay increase he will' is the wordplay.
'after' is a charade indicator (letters next to each other).
'pay increase' becomes 'raise' (I've seen this in another clue).
'will' becomes ''ll' (contracted form, as in 'he'll' - 'he will').
'raise' next to 'he' is 'raisehe'.
'raisehe'+'ll'='RAISE HELL'
(Other definitions for raise hell that I've seen before include "create commotion" , "Make trouble" , "cause trouble" , "Complain in the strongest possible terms" , "create discord" .)