Broadcast medium mostly one that’s a spokesperson’s business? (5)
I believe the answer is:
radii
'that's a spokesperson's business?' is the definition.
(plural of 'radius', which can mean a spoke)
'broadcast medium mostly one' is the wordplay.
'broadcast medium' becomes 'radio' (radio is a broadcast medium).
'mostly' means to remove the last letter.
'one' becomes 'i' (Roman numeral).
'radio' with its last letter taken off is 'radi'.
'radi'+'i'='RADII'
(Other definitions for radii that I've seen before include "Lines drawn from a circle's centre" , "Lines from a circle's centre to the circumference" , "Two making up diameter" , "parts for arms" , "spoke with others" .)