Bring back controls on time at start of exam (9)
I believe the answer is:
reinstate
'bring back' is the definition.
(I've seen this before)
'controls on time at start of exam' is the wordplay.
'controls' becomes 'reins' (rein is a kind of control).
'on' says to put letters next to each other.
'time' becomes 't' (abbreviation).
'start of' indicates taking the first letters.
The initial letter of 'exam' is 'e'.
'reins'+'t'+'at'+'e'='REINSTATE'
(Other definitions for reinstate that I've seen before include "Put back into office" , "Restore to former post" , "Restore, put back" , "Restore to previous state or rank" , "Re-establish" .)