Coach amused stinging swimmer, one of 21 (6)
I believe the answer is:
Here is my best explanation:
'stinging swimmer one of 21' is the definition.
I can't judge whether this definition defines the answer.
'coach amused' is the subsidiary indication.
'coach' is an anagram indicator (coach can mean to change or improve).
'amused' is an anagram of 'medusa'.
(Other definitions for medusa that I've seen before include "Jellyfish; gorgon", "Mythical woman with snakes for hair", "Amused like Greek Gorgon", "Gorgon of Greek myth", "Mythological gorgon", "Gorgon slain by Perseus", "Form of jellyfish", "Amused (anag)", "Hideous female" and "Petrifying woman".)