Scottish John stops Pilot Officer in quiet passage (5)
I believe the answer is:
piano
'quiet passage' is the definition.
(I've seen this before)
'scottish john stops pilot officer' is the wordplay.
'scottish john' becomes 'ian' (I've seen this in another clue).
'stops' is an insertion indicator (stopping can mean blocking a gap).
'pilot officer' becomes 'po' (short for pilot officer).
'ian' going into 'po' is 'PIANO'.
'in' acts as a link.
(Other definitions for piano that I've seen before include "Keyboard instrument" , "one of 6 [STEINWAYS]" , "Instrument with ivories" , "In music, soflty" , "Quiet (music)" .)