Commander conceals socialist beliefs (5)
I believe the answer is:
credo
'beliefs' is the definition.
(I've seen this before)
'commander conceals socialist' is the wordplay.
'commander' becomes 'co' (short for commanding officer).
'conceals' is an insertion indicator.
'socialist' becomes 'red' (socialism is often associated with the colour red).
'co' going around 'red' is 'CREDO'.
(Other definitions for credo that I've seen before include "Statement of guiding aims" , "Words of believer" , "ideology" , "Doctrine or belief" , "A statement of one's beliefs" .)