Keep mum confined (4,2)
I believe the answer is:
shut up
This clue is a double definition.
'keep mum' is the first definition.
(synonyms)
'confined' is the second definition.
(similar in meaning)
(Other definitions for shut up that I've seen before include "Keep mum" , "say nothing" , "Close your mouth!" , "Hold your tongue" , "Closely confined" .)