Opening left wall exposed (6)
I believe the answer is:
portal
'opening' is the definition.
(type of opening or entrance)
'left wall exposed' is the wordplay.
'left' becomes 'port' (left side of a ship - opposite of starboard).
'exposed' means to look at the middle letters (the inside of the word is exposed).
The centre of 'wall' is 'al'.
'port'+'al'='PORTAL'
(Other definitions for portal that I've seen before include "Grand sort of entrance." , "Special website bringing together information from diverse sources in a uniform way" , "Grand and imposing entrance" , "An imposing gate or doorway" , "'Doorway, entrance (6)'" .)