Expression of amusement having left lounge (4)
I believe the answer is:
loll
'lounge' is the definition.
(thesaurus)
'expression of amusement having left' is the wordplay.
'expression of amusement' becomes 'lol' (abbreviation for 'laugh out loud').
'having' means one lot of letters go next to another.
'left' becomes 'l' (common abbreviation).
'lol'+'l'='LOLL'
(Other definitions for loll that I've seen before include "Lean back in lazy and relaxed manner" , "lie and take it easy" , "Lie sprawled" , "Hang loose" , "Slouch" .)