Lap speed that’s fast (4)
I believe the answer is:
lick
'lap' is the definition.
('lick' can be a synonym of 'lap')
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for lick that I've seen before include "(At a) fast pace - beat conclusively" , "Defeat (informal) - speed (informal)" , "Beat; vigorous speed" , "Lap - speed" , "Take in with tongue" .)