Key passage heard (4)
I believe the answer is:
isle
'key' is the definition.
(I've seen this before)
'passage heard' is the wordplay.
'passage' becomes 'aisle' (aisle is a kind of passage).
'heard' shows a homophone (sound like).
'aisle' is a homophone of 'ISLE'.
(Other definitions for isle that I've seen before include "Sheppey, perhaps" , "Isolated land" , "Man, for one" , "Hawaii, perhaps" , "'This scepter'd - - (Shakespeare)" .)