Still cold wearing tartan (6)
I believe the answer is:
placid
'still' is the definition.
(I know that still can be written as placid)
'cold wearing tartan' is the wordplay.
'cold' becomes 'c' (eg on taps).
'wearing' is an insertion indicator.
'tartan' becomes 'plaid' (synonyms).
'c' inserted within 'plaid' is 'PLACID'.
(Other definitions for placid that I've seen before include "Composed or serene" , "Serene - unruffled" , "Peaceable and not easily disturbed" , "Imperturbable" , "Calm and still" .)