Unruffled tartan worn by chief of clan (6)
I believe the answer is:
placid
'unruffled' is the definition.
('placid' can be a synonym of 'unruffled')
'tartan worn by chief of clan' is the wordplay.
'tartan' becomes 'plaid' ('plaid' can be a synonym of 'tartan').
'worn by' indicates putting letters inside (I've seen this in other clues).
'chief of' says to take the initial letters.
The initial letter of 'clan' is 'c'.
'plaid' enclosing 'c' is 'PLACID'.
(Other definitions for placid that I've seen before include "Calm, unruffled" , "Not ruffled" , "Quiet and still" , "Calm and free from disturbance" , "Composed or serene" .)