Keep mother in check (6)
I believe the answer is:
Here is my best explanation:
'keep' is the definition.
'mother in check' is the wordplay.
'mother' becomes 'ma' (ma is a kind of mother).
'in' is an insertion indicator.
'check' becomes 'rein' (both can mean to control or limit).
'ma' put inside 'rein' is 'REMAIN'.
(Other definitions for remain that I've seen before include "Stand", "Stay on after others have gone", "Stay, don't go", "Stay (in a place)", "Stay behind", "Be left", "Stay around", "Continue in position", "Airmen (anag.)" and "Stay or continue to be".)