The entrance is left at the top of the alley (6)
I believe the answer is:
portal
'the entrance' is the definition.
(portal is a kind of entrance)
'left at the top of the alley' is the wordplay.
'left' becomes 'port' (left side of a ship - opposite of starboard).
'at' is a charade indicator (letters next to each other).
'the top of the alley' becomes 'al' (I can't justify this - if you can you should give a lot more credence to this answer).
'port'+'al'='PORTAL'
'is' is the link.
Can you help me to learn more?
(Other definitions for portal that I've seen before include "Special website bringing together information from diverse sources in a uniform way" , "An imposing gate or doorway" , "An imposing door or gate" , "Gate, entrance" , "Grand and imposing entrance" .)