Girl has upset a boy (6)
I believe the answer is:
sharon
'girl' is the definition.
'has upset a boy' is the wordplay.
'upset' indicates an anagram.
'a boy' becomes 'ron' (short for Ronald).
'has' is an anagram of 'sha'.
'sha'+'ron'='SHARON'
(Other definitions for sharon that I've seen before include "Girl's name; -- fruit" , "Girl's name (with rose)" , "Ariel ..., former prime minister of Israel" .)