Lunatic I cannot restrain (7)
I believe the answer is:
contain
'restrain' is the definition.
('contain' can be a synonym of 'restrain')
'lunatic i cannot' is the wordplay.
'lunatic' indicates anagramming the letters.
'i'+'cannot'='icannot'
'icannot' with letters rearranged gives 'CONTAIN'.
(Other definitions for contain that I've seen before include "Keep within limits or have within" , "Include - restrict" , "Accommodate" , "Keep the lid on" , "keep within bounds" .)