Snake in sea speeding (3)
I believe the answer is:
asp
'snake' is the definition.
(asp is a kind of snake)
'in sea speeding' is the wordplay.
'in' indicates the answer is hidden within the clue.
'ASP' can be found hidden inside 'sea speeding'.
(Other definitions for asp that I've seen before include "Cleopatra-killing snake" , "creeper" , "snake appears" , "Cleopatra's supposed nemesis" , "Slithery thing" .)