Warning from revolutionary clandestine movement (4)
I believe the answer is:
omen
'warning' is the definition.
(I've seen this before)
'from revolutionary clandestine movement' is the wordplay.
'revolutionary' says the letters should be written backwards.
'from' indicates the answer is hidden within the clue.
'nemo' is hidden within 'clandestine movement'.
'nemo' backwards is 'OMEN'.
(Other definitions for omen that I've seen before include "Forecast" , "Portent, prophetic indicator" , "writing on the wall" , "Harbinger, augury" , "Sign of likely future event" .)