"Right on!" (3)
'right on' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for amen that I've seen before include "Word of approval" , "Churchgoer's assent?" , "Response to prayer" , "Prayerful last word" , "Extremist" .)
(Other definitions for rad that I've seen before include "measure of dangerous activity" , "Unit of radiation" .)