Left over and under a gate (6)
I believe the answer is:
portal
'gate' is the definition.
(thesaurus)
'left over' is the wordplay.
'left' becomes 'port' (left side of a ship - opposite of starboard).
'over' becomes 'al' (I am not sure about this - if you are sure you should believe this answer much more).
'port'+'al'='PORTAL'
'and under a' is the link.
This may not be right. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for portal that I've seen before include "An imposing gateway" , "An imposing door or gate" , "Gate, entrance" , "An imposing gate or doorway" , "Special website bringing together information from diverse sources in a uniform way" .)