Opening left cavity (8)
I believe the answer is:
porthole
'opening' is the definition.
(porthole is a kind of opening)
'left cavity' is the wordplay.
'left' becomes 'port' (left side of a ship - opposite of starboard).
'cavity' becomes 'hole' (hole is a kind of cavity).
'port'+'hole'='PORTHOLE'
(Other definitions for porthole that I've seen before include "Small watertight opening in ship's side" , "from which sea can be seen" , "Circular window in ship" , "Ship's window" , "Window on board ship" .)