Leader of clan in tartan, serene (6)
I believe the answer is:
placid
'serene' is the definition.
('placid' can be a synonym of 'serene')
'leader of clan in tartan' is the wordplay.
'leader of' indicates taking the first letters.
'in' is an insertion indicator.
'tartan' becomes 'plaid' ('plaid' can be a synonym of 'tartan').
The first letter of 'clan' is 'c'.
'c' inserted inside 'plaid' is 'PLACID'.
(Other definitions for placid that I've seen before include "Peaceable and not easily disturbed" , "or is it an American one?" , "Calm, unruffled" , "Calm; equable" , "mild" .)