Objection about locking up opponents began again (7)
I believe the answer is:
resumed
'began again' is the definition.
(to resume is to start doing something again)
'objection about locking up opponents' is the wordplay.
'objection' becomes 'demur' (demur is a kind of objection).
'about' says the letters should be written in reverse.
'locking up' is an insertion indicator (I've seen this in other clues).
'opponents' becomes 'es' (I've seen this in other clues).
'demur' backwards is 'rumed'.
'rumed' placed around 'es' is 'RESUMED'.
(Other definitions for resumed that I've seen before include "Began again" , "Restarted" , "Recommenced" , "Once again picked up" , "got to work on again" .)