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' (ma can mean mother).
'in' is an insertion indicator.
'check' becomes 'rein' (both can mean to control or limit).
'ma' inserted inside '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" .)