Key left after move (8)
I believe the answer is:
passport
'key' is the definition.
(both can mean a means of accessing something)
'left after move' is the wordplay.
'left' becomes 'port' (left side of a ship - opposite of starboard).
'after' says to put letters next to each other.
'move' becomes 'pass' (passing is a kind of moving).
'port' put after 'pass' is 'PASSPORT'.
(Other definitions for passport that I've seen before include "Document facilitating travel" , "one admits" , "papers" , "Official document required for foreign travel" , "Travel doc" .)