Next in line making sound impression (4)
I believe the answer is:
'next in line' is the definition.
(I've seen this before)
'sound impression' is the wordplay.
'sound' shows a homophone (sound like).
'impression' becomes 'air' (I've seen this before).
'air' sounds like 'HEIR'.
'making' acts as a link.
(Other definitions for heir that I've seen before include "one's to inherit throne!" , "One hoping to succeed" , "Someone expected to inherit" , "One destined to succeed" , "Successor to position or estate" .)