Lady's dead posh and game (7)
I believe the answer is:
duchess
'lady's' is the definition.
(duchess is a kind of lady)
'dead posh and game' is the wordplay.
'dead' becomes 'd'.
'posh' becomes 'u' (abbreviation for upper-class - opposite to non-u).
'and' means one lot of letters go next to another.
'game' becomes 'chess' (chess is a kind of game).
'd'+'u'+'chess'='DUCHESS'
(Other definitions for duchess that I've seen before include "Female aristocrat" , "Titled female" , "Titled lady" , "Noblewoman" , "Noble lady" .)