Keep mother in check (6)
I believe the answer is:
Here is my best explanation:
'keep' is the definition.
'mother in check' is the subsidiary indication.
'mother' becomes 'ma' (ma is a kind of mother).
'in' means one lot of letters goes inside another.
'check' becomes 'rein' (reining is a kind of checking**).
'ma' inserted inside 'rein' is 'remain'.
(Other definitions for remain that I've seen before include "Stay, don't go", "Old dance", "Stay behind", "Airmen (anag.)", "Stay on when others have left", "Stay on after others have gone", "Marine (anag)", "Stay around", "Stay (in position)" and "Continue in position".)