Some radical politician, revolutionary having calm demeanour (6)
I believe the answer is:
placid
'having calm demeanour' is the definition.
'some radical politician revolutionary' is the wordplay.
'some' indicates a hidden word (some of the parts of the word are used).
'revolutionary' shows that the letters should be reversed in order.
'dicalp' is hidden within 'radical politician'.
'dicalp' in reverse letter order is 'PLACID'.
(Other definitions for placid that I've seen before include "Serene - unruffled" , "Tranquil, unruffled" , "Calm and still" , "Still - US lake" , "Peaceful - composed" .)