Three fellows drinking in bar (4)
I believe the answer is:
reef
'bar' is the definition.
(thesaurus)
'three fellows drinking in' is the wordplay.
'in' says the answer is hidden in the clue.
'REEF' is hidden in the letters of 'three fellows drinking'.
(Other definitions for reef that I've seen before include "Rocky ridge near sea surface" , "Atoll protector" , "Ridge of coral near the sea surface" , "Free (anag.)" , "Shallow coral ridge" .)