Passage requiring a key? (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' (I've seen this before).
'a'+'isle'='AISLE'
'requiring' is the link.
(Other definitions for aisle that I've seen before include "Part of supermarket" , "part of theatre?" , "A key" , "Passage down the side of a church nave" , "channel" .)