In three lanes, only one swimmer (3)
I believe the answer is:
eel
'one swimmer' is the definition.
(eel is a type of fish)
'in three lanes only' is the wordplay.
'in' indicates the answer is hidden within the clue.
'EEL' can be found hidden inside 'three lanes only'.
(Other definitions for eel that I've seen before include "Almost scaleless fish" , "Slippery creature" , "Creature like conger or moray" , "Sea creature like conger" , "Slippery swimmer (may be jellied)" .)