It's just one taking part in mail distribution (9)
I believe the answer is:
impartial
'it's just' is the definition.
(I've seen this before)
'one taking part in mail distribution' is the wordplay.
'one' becomes 'i' (Roman numeral**).
'taking' says to put letters next to each other.
'in' is an insertion indicator.
'distribution' indicates anagramming the letters.
'mail' anagrammed gives 'mial'.
'part' put into 'mial' is 'mpartial'.
'i'+'mpartial'='IMPARTIAL'
(Other definitions for impartial that I've seen before include "Unbiased" , "Fair, unprejudiced" , "Disinterested" , "not taking sides" , "Objective" .)