Works quietly to stop old habits (6)
I believe the answer is:
opuses
'works' is the definition.
(I've seen this before)
'quietly to stop old habits' is the wordplay.
'quietly' becomes 'p' (p for piano and quiet).
'to stop' means one lot of letters goes inside another.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'habits' becomes 'uses' ('use' can be a synonym of 'habit').
'o'+'uses'='ouses'
'p' put into 'ouses' is 'OPUSES'.
(Other definitions for opuses that I've seen before include "pieces of music" , "Musical works" .)