Tex moving round posh party in formal wear (6)
I believe the answer is:
tuxedo
'formal wear' is the definition.
(I've seen this in another clue)
'tex moving round posh party' is the wordplay.
'moving' is an anagram indicator.
'round' indicates putting letters inside.
'posh' becomes 'u' (abbreviation for upper-class - opposite to non-U).
'party' becomes 'do' (do is a kind of party).
'tex' anagrammed gives 'txe'.
'txe' placed around 'u' is 'tuxe'.
'tuxe'+'do'='TUXEDO'
'in' is the link.
(Other definitions for tuxedo that I've seen before include "Dinner jacket (US)" , "An American monkey-suit" , "Garment" , "American DJ" , "American dinner jacket" .)