Pupil once expelled after losing head complained (8)
I believe the answer is:
objected
'complained' is the definition.
(to complain about something is to object to it)
'pupil once expelled after losing head' is the wordplay.
'pupil once' becomes 'OB' (Old Boy can mean a former pupil of a school).
'expelled' becomes 'ejected' ('eject' can be a synonym of 'expel').
'after losing head' means to remove the first letter.
'ejected' with its initial letter taken away is 'jected'.
'ob'+'jected'='OBJECTED'
(Other definitions for objected that I've seen before include "Argued against" , "Expressed opposition" , "Protested" , "Refused assent" .)