Annoyed with a deserter failing to commence duty? (5)
I believe the answer is:
ratty
'annoyed' is the definition.
(thesaurus)
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for ratty that I've seen before include "Short" , "Characteristic of pest" , "Irritable (colloq.)" , "Moley's friend" , "Wretched" .)