Spy agreed to support revolutionary operations (5)
I believe the answer is:
spook
'spy' is the definition.
(spook can informally mean a spy)
'agreed to support revolutionary operations' is the wordplay.
'agreed' becomes 'ok' (eg when responding to an offer).
'to support' says to put letters next to each other (in a down clue, some letters hold others up).
'revolutionary' shows that the letters should be reversed in order.
'operations' becomes 'ops' (abbreviation).
'ops' written backwards gives 'spo'.
'ok' put after 'spo' is 'SPOOK'.
(Other definitions for spook that I've seen before include "Frighten - haunt - spy" , "agent" , "Ghost - US spy" , "Enterprise propels him" , "Ghost, spectre" .)