In the habit of playing duets with nothing on (4,2)
I believe the answer is:
used to
'in the habit of' is the definition.
(someone used to something is in the habit of it)
'playing duets with nothing on' is the wordplay.
'playing' indicates anagramming the letters.
'with' is a charade indicator (letters next to each other).
'nothing on' becomes 'o' (I've seen this in other clues).
'duets' anagrammed gives 'usedt'.
'usedt'+'o'='USED TO'
(Other definitions for used to that I've seen before include "Was in the habit" , "Accustomed or inured" , "Did at times in the past" , "Familiar with" , "Not unfamiliar with" .)