Appropriate whilst in transit (6)
I believe the answer is:
hijack
'appropriate whilst in transit' is the definition.
(appropriate meaning steal)
This is the entire clue.
(Other definitions for hijack that I've seen before include "Seize a vehicle forcefully" , "Take over forcibly" , "Seize a vehicle illegally" , "Take (aircraft) by force" , "Illegally seize control of (a plane/ship)" .)