Judge going to posh party for sport (4)
I believe the answer is:
judo
'for sport' is the definition.
(judo is a kind of sport)
'judge going to posh party' is the wordplay.
'judge' becomes 'J' (abbreviation for judge).
'going to' says to put letters next to each other.
'posh' becomes 'u' (abbreviation for upper-class - opposite to non-U).
'party' becomes 'do' (do can informally mean a party).
'j'+'u'+'do'='JUDO'
(Other definitions for judo that I've seen before include "Form of wrestling" , "Throwing sport (from the Japanese for gentleness)" , "Wrestling sport adapted from jujitsu" , "Martial art (not sumo)" , "Unarmed combat sport" .)