Sister, one kept in business as messenger (6)
I believe the answer is:
nuncio
'messenger' is the definition.
(thesaurus)
'sister one kept in business' is the wordplay.
'sister' becomes 'nun' (I've seen this before).
'one' becomes 'i' (Roman numeral).
'kept in' means one lot of letters goes inside another.
'business' becomes 'co'.
'i' placed inside 'co' is 'cio'.
'nun'+'cio'='NUNCIO'
'as' acts as a link.
(Other definitions for nuncio that I've seen before include "Papal envoy" , "See man" , "Papal emissary" , "Diplomatic representative of the Pope" , "Pope's diplomatic representative" .)