One with habit that's a bit flipping presumptuous (4)
I believe the answer is:
user
'one with habit' is the definition.
(someone with a drug habit)
'a bit flipping presumptuous' is the wordplay.
'flipping' shows that the letters should be reversed in order.
'a bit' indicates the answer is hidden within the clue (I've seen 'bit of' mean this).
'resu' can be found hidden inside 'presumptuous'.
'resu' in reverse letter order is 'USER'.
'that's' acts as a link.
(Other definitions for user that I've seen before include "Consumer" , "Operator; exploiter" , "Drug taker" , "One taking drugs" , "One operating something" .)