Commanding officer conceals revolutionary beliefs (5)
I believe the answer is:
credo
'beliefs' is the definition.
(I've seen this before)
'commanding officer conceals revolutionary' is the wordplay.
'commanding officer' becomes 'co'.
'conceals' indicates putting letters inside.
'revolutionary' becomes 'red'.
'co' enclosing 'red' is 'CREDO'.
(Other definitions for credo that I've seen before include "Part of the Latin Mass" , "mission statement" , "piece of sacred music" , "A statement of one's beliefs" , "System of principles" .)