Sex in party set up for poet (4)
I believe the answer is:
ovid
'for poet' is the definition.
(Roman poet)
'sex in party set up' is the wordplay.
'sex' becomes 'VI' (VI in Roman numerals - 'sex' is 'six' in Latin).
'in' indicates putting letters inside.
'party' becomes 'do' (do can informally mean a party).
'set up' shows that the letters should be reversed in order (in a down clue, letters go up).
'do' in reverse letter order is 'od'.
'vi' placed into 'od' is 'OVID'.
(Other definitions for ovid that I've seen before include "Roman poet (Metamorphoses)" , "Roman poet N void (anag)" , "Void for a Latin poet" , "Metamorphoses author" , "Old linesman" .)