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' indicates a hidden word.
'REEL' is hidden in the letters of '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" .)