Prepare to take off in cab (4)
I believe the answer is:
taxi
I believe this is a double definition.
'prepare to take off' is the first definition.
(a plane taxis along a runway before takeoff)
'cab' is the second definition.
(taxi cab)
'in' acts as a link.
(Other definitions for taxi that I've seen before include "Move along a runway" , "Transport for hire" , "Prepare to take off" , "Car for hire" , "At airport, guide" .)