Relating to pieces showing FBI agents had set about alarming leaders (9)
I believe the answer is:
segmental
'set about alarming leaders' is the definition.
Both the definition and answer are adjectives. Perhaps they are linked in a way I don't understand?
'relating to pieces showing fbi agents' is the wordplay.
I cannot quite see how this works, but
'pieces' could be 'men' (man can mean a chess piece) and 'men' is found within the answer.
This may be the basis of the clue (or it may be nonsense).
'had' acts as a link.
Can you help me to learn more?
(Other definitions for segmental that I've seen before include "As, say, Orange" , "Composed of distinct parts" .)