Prophet before noon so revolutionary (4)
I believe the answer is:
amos
'prophet' is the definition.
(biblical prophet)
'before noon so revolutionary' is the wordplay.
'before noon' becomes 'AM' (as in AM on a clock).
'revolutionary' says the letters should be written in reverse (letters revolve around).
'so' backwards is 'os'.
'am'+'os'='AMOS'
(Other definitions for amos that I've seen before include "Biblical character" , "Prophet in the Bible" , "Old Testament book" , "Biblical prophet and OT book" , "man" .)