Person on a soapbox (6)
I believe the answer is:
orator
'person on a soapbox' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for orator that I've seen before include "I declaim" , "One addressing" , "A gifted public speaker" , "Eloquent public speaker" , "One who holds forth" .)