A key passage (5)
I believe the answer is:
aisle
'passage' is the definition.
(aisle can mean a path or passageway)
'a key' is the wordplay.
'key' becomes 'isle' (key can mean a small island).
'a'+'isle'='AISLE'
(Other definitions for aisle that I've seen before include "Passage between shelves in supermarket" , "way for prospective union members?" , "part of theatre?" , "Way between seat blocks" , "Part of a church" .)