Jellyfish amused flounders (6)
I believe the answer is:
medusa
'jellyfish' is the definition.
(synonyms)
'amused flounders' is the wordplay.
'flounders' indicates anagramming the letters (I've seen 'floundering' mean this (letters flounder or move around)).
'amused' is an anagram of 'MEDUSA'.
(Other definitions for medusa that I've seen before include "Mythical woman transformed into a Gorgon" , "all her visitors got stoned" , "Gorgon killed by Perseus - jellyfish" , "Amused like Greek Gorgon" , "Mythical woman with snakes for hair" .)