Cool Conservative in tartan (6)
I believe the answer is:
placid
'cool' is the definition.
(I've seen this before)
'conservative in tartan' is the wordplay.
'conservative' becomes 'C' (UK political abbreviation).
'in' indicates putting letters inside.
'tartan' becomes 'plaid' (synonyms).
'c' put into 'plaid' is 'PLACID'.
(Other definitions for placid that I've seen before include "Still - US lake" , "Calm, peaceful" , "Quiet and still" , "Calm and free from disturbance" , "Calm of nature" .)