Annoys German aboard vehicle (4)
I believe the answer is:
bugs
'annoys' is the definition.
(I've seen this in another clue)
'german aboard vehicle' is the wordplay.
'german' becomes 'g' (abbreviation).
'aboard' indicates putting letters inside.
'vehicle' becomes 'bus' (bus is a kind of vehicle).
'g' put inside 'bus' is 'BUGS'.
(Other definitions for bugs that I've seen before include "Secret listening devices" , "Insects or faults in a system or machine" , "Small crawling creatures or disease-germs" , "Insects or defects in computer programs" , "Software problems" .)