Judge at posh party for sport (4)
I believe the answer is:
judo
'sport' is the definition.
(judo is a kind of sport)
'judge at posh party' is the wordplay.
'judge' becomes 'j'.
'at' means one lot of letters go next to another.
'posh' becomes 'u' (abbreviation for upper-class - opposite to non-u).
'party' becomes 'do' (do is a kind of party).
'j'+'u'+'do'='JUDO'
'for' is the link.
(Other definitions for judo that I've seen before include "Martial art (not sumo)" , "Martial art - Olympic sport" , "13 art of self-defence" , "Form of wrestling" , "Sport derived from jujitsu" .)