Sign of a posh zip (7)
I believe the answer is:
auspice
'sign' is the definition.
(auspice is a kind of sign)
'a posh zip' is the wordplay.
'posh' becomes 'u' (abbreviation for upper-class - opposite to non-U).
'zip' becomes 'spice' (both can mean excitement or energy).
'a'+'u'+'spice'='AUSPICE'
'of' acts as a link.
(Other definitions for auspice that I've seen before include "sponsorship" , "future promise" , "Sign" , "Omen (archaic)" , "Omen; epic USA (anag.)" .)