Keep mum in check (6)
I believe the answer is:
remain
'keep' is the definition.
(thesaurus)
'mum in check' is the wordplay.
'mum' becomes 'ma' (both can mean mother).
'in' indicates putting letters inside.
'check' becomes 'rein' (both can mean to control or limit).
'ma' placed within 'rein' is 'REMAIN'.
(Other definitions for remain that I've seen before include "Last" , "Marine (anag)" , "Old dance" , "stop behind?" , "Stay on after others have gone" .)