Head of couture in tartan, being cool (6)
I believe the answer is:
placid
'being cool' is the definition.
(I've seen this before)
'head of couture in tartan' is the wordplay.
'head of' suggests taking the first letters.
'in' is an insertion indicator.
'tartan' becomes 'plaid' ('plaid' can be a synonym of 'tartan').
The initial letter of 'couture' is 'c'.
'c' put inside 'plaid' is 'PLACID'.
(Other definitions for placid that I've seen before include "Peaceful - composed" , "without ruffles" , "Tranquil, unruffled" , "Calm and still" , "Composed or serene" .)