Swimmer’s incompetent first of four splits (9)

I believe the answer is:

clownfish

'swimmer's' is the definition.
(something which swims)

'incompetent first of four splits' is the wordplay.
'incompetent' becomes 'clownish' (clownish can mean inept).
'first of four' becomes 'f' (1st letter of 'four').
'splits' is an insertion indicator (inserted letters split open the word).
'clownish' going around 'f' is 'CLOWNFISH'.

I've seen this clue in The Times.

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