A single payment posh politician's hidden in chimney (4,3)
I believe the answer is:
lump sum
'a single payment' is the definition.
(I've seen this before)
'posh politician's hidden in chimney' is the wordplay.
'posh' becomes 'u' (abbreviation for upper-class - opposite to non-U).
'politician's' becomes 'MP's' (Member of Parliament).
'hidden in' indicates putting letters inside.
'chimney' becomes 'lum' (I have seen 'Scottish chimney ' mean 'lum' so perhaps 'chimney' could also mean 'lum').
'u'+'mps'='umps'
'umps' inserted within 'lum' is 'LUMP SUM'.
(Other definitions for lump sum that I've seen before include "cash payout" , "Single amount of money" , "Amount of money paid all at once" , "Single payment as opposed to a number of smaller ones" , "one lot of money" .)