Old principal spooks revolutionary (7)
I believe the answer is:
archaic
'old' is the definition.
(similar in meaning)
'principal spooks revolutionary' is the wordplay.
'principal' becomes 'arch' (as in arch-enemy).
'spooks' becomes 'CIA' (spooks can mean spies).
'revolutionary' says the letters should be written in reverse.
'cia' reversed gives 'aic'.
'arch'+'aic'='ARCHAIC'
(Other definitions for archaic that I've seen before include "Belonging to former times, perhaps word usage" , "Antiquarian" , "Outdated" , "Belonging to former times like some words" , "Ancient or out of date" .)