Girl in party upset old poet (4)
I believe the answer is:
ovid
'old poet' is the definition.
(I've seen this before)
'girl in party upset' is the wordplay.
'girl' becomes 'vi' (short for Violet).
'in' is an insertion indicator.
'party' becomes 'do' (do is a kind of party).
'upset' shows that the letters should be reversed in order.
'do' in reverse letter order is 'od'.
'vi' placed within 'od' is 'OVID'.
(Other definitions for ovid that I've seen before include "Vi, do read the Latin poet" , "Roman poet exiled by Augustus to Tomi on the Black Sea, where he died, AD 17" , "Void for old Roman poet" , "Latin poet (Ars amatoria, Metamorphoses)" , "Roman poet, d. about AD17" .)