Left aluminium in entrance (6)
I believe the answer is:
portal
'entrance' is the definition.
(type of opening or entrance)
'left aluminium' is the wordplay.
'left' becomes 'port' (left side of a ship - opposite of starboard).
'aluminium' becomes 'al' (Al is the chemical symbol for aluminium).
'port'+'al'='PORTAL'
'in' acts as a link.
(Other definitions for portal that I've seen before include "Grand sort of entrance." , "An imposing gate or doorway" , "Grand and imposing entrance" , "An imposing gateway" , "An imposing door or gate" .)