Still Caledonian's first dressed in tartan (6)
I believe the answer is:
placid
'still' is the definition.
('placid' can be a synonym of 'still')
'caledonian's first dressed in tartan' is the wordplay.
'first' indicates taking the first letters.
'dressed in' means one lot of letters goes inside another.
'tartan' becomes 'plaid' ('plaid' can be a synonym of 'tartan').
The first letter of 'caledonians' is 'c'.
'c' placed within 'plaid' is 'PLACID'.
(Other definitions for placid that I've seen before include "Unexcitable" , "Calm, peaceful" , "Calm and free from disturbance" , "Cool and unruffled" , "without ruffles" .)