Posed with vase, famous boxer before a festival (10)
I believe the answer is:
saturnalia
'festival' is the definition.
(I know that Saturnalia is a type of festival)
'posed with vase famous boxer before a' is the wordplay.
'posed' becomes 'sat' (sitting is a kind of posing).
'with' says to put letters next to each other.
'vase' becomes 'urn' (both can mean a container).
'famous boxer' becomes 'ali' (I've seen this in another clue).
'before' is a charade indicator (letters next to each other).
'sat'+'urn'+'ali'+'a'='SATURNALIA'
(Other definitions for saturnalia that I've seen before include "Old festival" , "wild party" , "orgy" , "revels" , "Wild Roman festival" .)