In tartan, the leader of the clan is unruffled (6)
I believe the answer is:
placid
'is unruffled' is the definition.
(synonyms)
'in tartan the leader of the clan' is the wordplay.
'in' is an insertion indicator.
'tartan' becomes 'plaid' (synonyms).
'the leader of' indicates taking the first letters.
The first letter of 'clan' is 'c'.
'plaid' placed around 'c' is 'PLACID'.
(Other definitions for placid that I've seen before include "Undisturbed" , "Calm, unruffled" , "Peaceful - composed" , "Calm, peaceful" , "Calm and free from disturbance" .)