She upsets me mum! (4)
I believe the answer is:
emma
'she' is the definition.
(I've seen this before)
'upsets me mum' is the wordplay.
'upsets' says the letters should be written backwards.
'mum' becomes 'ma' (ma can mean mother).
'me' back-to-front is 'em'.
'em'+'ma'='EMMA'
(Other definitions for emma that I've seen before include "Flaubert's heroine" , "It's a girl - ask me ma" , "Title of novel by Jane Austen" , "Madame Bovary, perhaps" , "novelist's work" .)