Knowing tune's picked up with habitual response (7)
I believe the answer is:
gnostic
'knowing' is the definition.
(I've seen this in another clue)
'tune's picked up with habitual response' is the wordplay.
'tune' becomes 'song' (I've seen this before).
'picked up' says the letters should be written backwards.
'with' means one lot of letters go next to another.
'habitual response' becomes 'tic' (an involuntary physical movement).
'song' written backwards gives 'gnos'.
'gnos'+'tic'='GNOSTIC'
(Other definitions for gnostic that I've seen before include "Having special mystical knowledge, costing a lot" , "Occult" , "Relating to mystical knowledge" .)