Willing to upset mage (4)
I believe the answer is:
game
'willing' is the definition.
(game can mean willing to do something)
'upset mage' is the wordplay.
'upset' indicates an anagram.
'mage' anagrammed gives 'GAME'.
'to' acts as a link.
(Other definitions for game that I've seen before include "See 3" , "See 8" , "Willing; recreation" , "See 21" , "Final declaration of tennis result - . . . ., set and match" .)