Beer losing head: nothing to get enthusiastic about (4)
I believe the answer is:
into
'enthusiastic about' is the definition.
(into can mean interested in or enthusiastic about)
'beer losing head nothing' is the wordplay.
'beer' becomes 'pint' (beer is typically served by the pint).
'losing head' means to remove the first letter.
'nothing' becomes 'O' (O resembles 0).
'pint' with its initial letter removed is 'int'.
'int'+'o'='INTO'
'to get' acts as a link.
(Other definitions for into that I've seen before include "Enthusiastic about" , "Keen on, colloquially" , "Teachers' trade union" , "Not I (anag.)" , "To a position inside" .)