Happen to restrain attendant, resulting in injury (5,3)
I believe the answer is:
black eye
'resulting in injury' is the definition.
'happen to restrain attendant' is the wordplay.
'happen' becomes 'be'.
'to restrain' indicates putting letters inside.
'attendant' becomes 'lackey'.
'be' placed around 'lackey' is 'BLACK EYE'.
(Other definitions for black eye that I've seen before include "Bruised facial area" , "Bruised optical area due to blow" , "result of being punched?" , "Colourful optic following blow" , "Kind of bruise - kind of bean" .)