Balance underpins diving position for swimmer (9)
I believe the answer is:
pikeperch
'swimmer' is the definition.
(pikeperch is a kind of fish)
'balance underpins diving position' is the wordplay.
'balance' becomes 'perch' (I am not sure about this - if you are sure you should believe this answer much more).
'underpins' is a charade indicator (letters next to each other) (in a down clue, some letters go underneath others).
'diving position' becomes 'pike' (I've seen this in another clue).
'perch' after 'pike' is 'PIKEPERCH'.
'for' acts as a link.
Can you help me to learn more?
(Another definition for pikeperch that I've seen is " Fish".)