Swimmer, after split, ahead on the turn! (4)
I believe the answer is:
tuna
'swimmer' is the definition.
(tuna is a type of fish)
'after split ahead on the turn' is the wordplay.
'after split' becomes 'na' (I can't explain this - if you can you should give a lot more credence to this answer).
'ahead' means one lot of letters go next to another.
'on' means one lot of letters go next to another.
'the' becomes 't' (the is pronounced as a 't' sound in some dialects).
'turn' becomes 'U' (a 'U-turn' is a kind of turn).
't' next to 'u' is 'tu'.
'na' put after 'tu' is 'TUNA'.
Can you help me to learn more?
(Other definitions for tuna that I've seen before include "Marine food fish" , "Large food fish" , "Skipjack --, blue-fin --" , "Yellow-finned fish" , "swimmer" .)