Indulge in drink, losing head (6)
I believe the answer is:
wallow
'indulge' is the definition.
(I know that wallow is a more specific form of the action indulge)
'drink losing head' is the wordplay.
'drink' becomes 'swallow' (swallow a drink).
'losing head' means to remove the first letter.
'swallow' with its initial letter taken away is 'WALLOW'.
'in' is the link.
(Other definitions for wallow that I've seen before include "Indulge excessively, in self-pity perhaps" , "Roll around as in mud" , "Lie about in mud or water" , "Luxuriate (in mud, muck)" , "Roll about as in water or mud" .)