Which swimmer is flushing badly? (8)
'which swimmer' is the definition.
I can't judge whether this definition defines the answer.
'flushing badly?' is the wordplay.
'badly?' is an anagram indicator (I've seen 'badly' mean this).
'flushing' with letters rearranged gives 'LUNGFISH'.
'is' acts as a link.
Can you help me to learn more?
(Another definition for lungfish that I've seen is "denizens of the river".)