Thus Ian upset the girl (5)
I believe the answer is:
sonia
'girl' is the definition.
(woman's name)
'thus ian upset' is the wordplay.
'thus' becomes 'so' (synonyms).
'upset' is an anagram indicator.
'ian' with letters rearranged gives 'nia'.
'so'+'nia'='SONIA'
'the' acts as a link.
(Other definitions for sonia that I've seen before include "Noa is really a girl" , "...O'Sullivan, great Irish athlete" .)