Passage is guarded by porter, say (5)
I believe the answer is:
aisle
'passage' is the definition.
(aisle is a kind of passage)
'is guarded by porter say' is the wordplay.
'guarded by' indicates putting letters inside.
'porter say' becomes 'ale' (porter is a kind of ale).
'is' put within 'ale' is 'AISLE'.
(Other definitions for aisle that I've seen before include "Passageway in theatre" , "Supermarket passageway" , "Part of supermarket" , "Passageway between pews" , "Extension of the nave in a church" .)