Polite and domestic - it's official (5,7)
I believe the answer is:
civil servant
'official' is the definition.
(I know that civil servant is a type of official)
'polite and domestic' is the wordplay.
'polite' becomes 'civil' (synonyms).
'and' is a charade indicator (letters next to each other).
'domestic' becomes 'servant' (I've seen this before).
'civil'+'servant'='CIVIL SERVANT'
'it's' acts as a link.
(Other definitions for civil servant that I've seen before include "Government employee" , "Government official" , "Person working in the public sector" .)