Troy upset novice (4)
I believe the answer is:
tyro
'novice' is the definition.
(term for a beginner)
'troy upset' is the wordplay.
'upset' indicates anagramming the letters.
'troy' anagrammed gives 'TYRO'.
(Other definitions for tyro that I've seen before include "Novice; Tory (anag.)" , "'Beginner, novice (4)'" , "Spell for rookie" , "Neophyte" , "Initiate" .)