Sister company admits one messenger (6)
I believe the answer is:
nuncio
'messenger' is the definition.
(thesaurus)
'sister company admits one' is the wordplay.
'sister' becomes 'nun' (I've seen this before).
'company' becomes 'co' (abbreviation).
'admits' is an insertion indicator.
'one' becomes 'i' (Roman numeral).
'co' placed around 'i' is 'cio'.
'nun'+'cio'='NUNCIO'
(Other definitions for nuncio that I've seen before include "Diplomatic representative of the Pope" , "Papal emissary" , "Papal diplomat" , "Pope's diplomatic representative" , "Papal messenger" .)