Girl upset with a fan’s informal language (9)
I believe the answer is:
franglais
'informal language' is the definition.
(a mixture of English and French)
'girl upset with a fan's' is the wordplay.
'upset with' indicates an anagram.
'a'+'fans'='afans'
'girl' anagrammed with 'afans' makes 'FRANGLAIS'.
(Other definitions for franglais that I've seen before include "Blended language" , "A mix of tongues" , "Corrupt language" , "informal version of language" , ""Je suis happy", say" .)