Left the beer unfinished, at the entrance (6)
I believe the answer is:
portal
'the entrance' is the definition.
(type of opening or entrance)
'left the beer unfinished' is the wordplay.
'left' becomes 'port' (left side of a ship - opposite of starboard).
'the beer' becomes 'ale' (type of beer).
'unfinished' means to remove the last letter.
'ale' with its last letter removed is 'al'.
'port'+'al'='PORTAL'
'at' is the link.
(Other definitions for portal that I've seen before include "'Doorway, entrance (6)'" , "Grand and imposing entrance" , "Grand sort of entrance." , "An imposing gateway" , "Gate, entrance" .)