Head of clan wearing tartan still (6)
I believe the answer is:
placid
'still' is the definition.
(I know that still can be written as placid)
'head of clan wearing tartan' is the wordplay.
'head of' suggests taking the first letters.
'wearing' is an insertion indicator.
'tartan' becomes 'plaid' (synonyms).
The initial letter of 'clan' is 'c'.
'c' put within 'plaid' is 'PLACID'.
(Other definitions for placid that I've seen before include "Quiet and still" , "Still - US lake" , "Calm, phlegmatic" , "Unconcerned" , "Tranquil, even-tempered" .)