A pay increase to happen (5)
I believe the answer is:
arise
'happen' is the definition.
(arising is a kind of happening)
'a pay increase' is the wordplay.
'pay increase' becomes 'rise' (I've seen this before).
'a'+'rise'='ARISE'
'to' acts as a link.
(Other definitions for arise that I've seen before include "Crop up" , "Ascend" , "Become apparent" , "Come to light" , "Wake up or get up" .)