Establishing bad habit in the sack (6)
I believe the answer is:
basing
'establishing' is the definition.
('base' can be a synonym of 'establish')
'bad habit in the sack' is the wordplay.
I cannot really see how this works, but
'bad' could be 'sin' and 'sin' is found within the answer.
'sack' could be 'bag' (I've seen this before) and 'bag' is present in the remaining letters.
No letters remain.
This explanation may well be incorrect...
Can you help me to learn more?
(Another definition for basing that I've seen is " Founding (on); stationing (at)".)