Expression of amusement heard in lounge (4)
I believe the answer is:
loll
'lounge' is the definition.
(both can mean to sit in a relaxed way)
'expression of amusement heard' is the wordplay.
'expression of amusement' becomes 'lol' (abbreviation for 'laugh out loud').
'heard' indicates a 'sounds like' (homophone) clue.
'lol' sounds like 'LOLL'.
'in' acts as a link.
(Other definitions for loll that I've seen before include "adopt a relaxed position" , "Hang loosely; sit lazily" , "Lean back in lazy and relaxed manner" , "Lounge around" , "Slouch" .)