Bad habit involving duck, say (5)
I believe the answer is:
voice
'say' is the definition.
(voicing is a kind of saying)
'bad habit involving duck' is the wordplay.
'bad habit' becomes 'vice' (I've seen this before).
'involving' is an insertion indicator.
'duck' becomes 'o'.
'vice' placed around 'o' is 'VOICE'.
(Other definitions for voice that I've seen before include "Sound with which one speaks" , "Power of speech" , "Stated opinion" , "tenor, say" , "Give expression to" .)