Trucker initially comes in to secure vehicle (5)
I believe the answer is:
motor
'vehicle' is the definition.
(motor vehicle is a kind of vehicle)
'trucker initially comes in to secure' is the wordplay.
'initially' says to take the initial letters.
'comes in to' means one lot of letters goes inside another.
'secure' becomes 'moor' (mooring is a kind of securing).
The first letter of 'trucker' is 't'.
't' put inside 'moor' is 'MOTOR'.
(Other definitions for motor that I've seen before include "go quickly" , "Car; powered machine" , "(Transport using) engine" , "Machine that converts energy into motion" , "Car (informal)" .)