Calm Charlie in tartan (6)
I believe the answer is:
placid
'calm' is the definition.
(synonyms)
'charlie in tartan' is the wordplay.
'charlie' becomes 'c' (phonetic alphabet: alpha, bravo, charlie etc.).
'in' indicates putting letters inside.
'tartan' becomes 'plaid' (synonyms).
'c' placed within 'plaid' is 'PLACID'.
(Other definitions for placid that I've seen before include "Peaceful - composed" , "Cool and unruffled" , "Unconcerned" , "Tranquil, even-tempered" , "Imperturbable" .)