Observe missing Northern Ireland vehicle (5)
I believe the answer is:
motor
'vehicle' is the definition.
(motor vehicle is a kind of vehicle)
'observe missing northern ireland' is the wordplay.
'observe' becomes 'monitor' (monitoring is a kind of observing).
'missing' is a deletion indicator.
'northern ireland' becomes 'ni'.
'monitor' with 'ni' removed is 'MOTOR'.
(Other definitions for motor that I've seen before include "Car (colloq,)" , "Move quickly" , "Move fast" , "Drive" , "Machine that converts energy into motion" .)