Undiluted sound in passage (6)
I believe the answer is:
strait
'passage' is the definition.
(I've seen this before)
'undiluted sound' is the wordplay.
'undiluted' becomes 'straight' (similar in meaning).
'sound' indicates a 'sounds like' (homophone) clue.
'straight' sounds like 'STRAIT'.
'in' acts as a link.
(Other definitions for strait that I've seen before include "Narrow stretch of sea" , "Difficulty - strict" , "Narrow sort of traits (6)" , "Inlet" , "Narrow sorts of traits" .)