Annoys German boarding aircraft (4)
I believe the answer is:
bugs
'annoys' is the definition.
(I've seen this in another clue)
'german boarding aircraft' is the wordplay.
'german' becomes 'g' (abbreviation).
'boarding' means one lot of letters goes inside another.
'aircraft' becomes 'bus' (I am not sure about this - if you are sure you should believe this answer much more).
'g' put inside 'bus' is 'BUGS'.
Can you help me to learn more?
(Other definitions for bugs that I've seen before include "Insects or faults in a system or machine" , "Secret listening devices" , "Errors in computer programs" , "Creepy-crawlies in computers" , "Small crawling creatures or disease-germs" .)