Plain clothes men, indeed, are emphatic (7)
I believe the answer is:
decided
'emphatic' is the definition.
(thesaurus)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for decided that I've seen before include "Marked" , "Indisputable" , "Resolute; settled" , "Definite; agreed" , "Resolved, settled" .)