Revolutionary man is under cover briefly (9)
I believe the answer is:
insurgent
'cover briefly' is the definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.
'revolutionary man is under' is the wordplay.
'revolutionary' indicates anagramming the letters.
'man' becomes 'N' (knight, a man in chess).
'under' becomes 'urgent' (I can't justify this - if you can you should believe this answer much more).
'n'+'is'='nis'
'nis' with letters rearranged gives 'ins'.
'ins'+'urgent'='INSURGENT'
Can you help me to learn more?
(Other definitions for insurgent that I've seen before include "Rebel, revolutionary" , "Unresting like a rebel" , "freedom fighter" , "rising" , "Revolting" .)