One who helps a posh couple? (2,4)
I believe the answer is:
au pair
'one who helps' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'a posh couple?' is the wordplay.
'posh' becomes 'u' (abbreviation for upper-class - opposite to non-u).
'couple?' becomes 'pair' (couple is a kind of pair).
'a'+'u'+'pair'='AU PAIR'
(Other definitions for au pair that I've seen before include "Foreign home helper" , "Working lodger" , "Living en famille" , "child-minder" , "does she work for board?" .)