Old money key upper-class party (6)
I believe the answer is:
escudo
'old money' is the definition.
(I've seen this before)
'key upper-class party' is the wordplay.
'key' becomes 'esc' ('Escape' key on a computer keyboard).
'upper-class' becomes 'u' (similar in meaning).
'party' becomes 'do' (do can informally mean a party).
'esc'+'u'+'do'='ESCUDO'
(Other definitions for escudo that I've seen before include "Former Portuguese currency unit" , "Old unit of money in Portugal" , "Old monetary unit of Portugal" , "money in past" , "Formerly the monetary unit of Portugal" .)