Constant exercises put on to keep wife quiet (4,4)
I believe the answer is:
pipe down
'quiet' is the definition.
(synonyms)
'constant exercises put on to keep wife' is the wordplay.
'constant' becomes 'pi' (mathematics: n.d4).
'exercises' becomes 'pe' (Physical Education class).
'put on' becomes 'don' (as in donning or putting on an item of clothing).
'to keep' is an insertion indicator.
'wife' becomes 'w'.
'don' placed around 'w' is 'down'.
'pi'+'pe'+'down'='PIPE DOWN'
(Other definitions for pipe down that I've seen before include "Sh!" , "Shut up!" , "Say no more!" , "Stop talking" , "make less noise" .)