Not so drunk, being accustomed to it (4)
I believe the answer is:
used
'accustomed to it' is the definition.
'used' can be an answer for 'accustomed' (I've seen this before). I am not sure about the 'to it' bit.
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for used that I've seen before include "Subject to exploitation" , "taken advantage of!" , "Pre-owned" , "Brought into service" , "Dues (anag.)" .)