Takes drugs and ponders when out of head (4)
I believe the answer is:
uses
'takes drugs' is the definition.
(someone who takes drugs is a 'user')
'ponders when out of head' is the wordplay.
'ponders' becomes 'muses' ('muse' can be a synonym of 'ponder').
'when out of head' means to remove the first letter.
'muses' with its initial letter taken away is 'USES'.
'and' is the link.
(Other definitions for uses that I've seen before include "Consumes; treats" , "'Purposes, functions (4)'" , "manipulates" , "Sues (anag.)" , "Utilises" .)