Sound immediately heard (6)
I believe the answer is:
strait
'sound' is the definition.
(I know that sound can be written as strait)
'immediately heard' is the wordplay.
'immediately' becomes 'straight' (as in straight away).
'heard' indicates a 'sounds like' (homophone) clue.
'straight' is a homophone of 'STRAIT'.
(Other definitions for strait that I've seen before include "Sound" , "Difficulty - strict" , "Narrow sorts of traits" , "Narrow stretch of water for artist" , "Narrow or difficult" .)