Swim in three lanes (4)
I believe the answer is:
reel
'swim' is the definition.
(I've seen this before)
'in three lanes' is the wordplay.
'in' says the answer is hidden in the clue.
'REEL' is hidden within 'three lanes'.
(Other definitions for reel that I've seen before include "Stagger with rod winder" , "Fishing rod attachment" , "spin" , "Lively Scottish dance" , "The angler pulls in his line using this" .)