Be all upset by a girl (5)
I believe the answer is:
bella
'a girl' is the definition.
'be all upset' is the wordplay.
'upset' is a reversal indicator.
'all' reversed gives 'lla'.
'be'+'lla'='BELLA'
'by' acts as a link.
(Other definitions for bella that I've seen before include "Girl's name" , "Girl" , "Deadly nightshade or atropine" .)