Still clan's leader wears tartan (6)
I believe the answer is:
placid
'still' is the definition.
(synonyms)
'clan's leader wears tartan' is the wordplay.
'leader' indicates taking the first letters.
'wears' indicates putting letters inside.
'tartan' becomes 'plaid' (synonyms).
The first letter of 'clans' is 'c'.
'c' going into 'plaid' is 'PLACID'.
(Other definitions for placid that I've seen before include "Quiet and still" , "Calm and tranquil" , "Cool and unruffled" , "mild" , "Calm, unruffled" .)