Serene number in tartan (6)
I believe the answer is:
placid
'serene' is the definition.
(synonyms)
'number in tartan' is the wordplay.
'number' becomes 'C' (C is an example).
'in' means one lot of letters goes inside another.
'tartan' becomes 'plaid' (synonyms).
'c' going within 'plaid' is 'PLACID'.
(Other definitions for placid that I've seen before include "Calm; equable" , "Tranquil, unruffled" , "Peaceful and unruffled" , "Not easily upset or excited" , "Unexcitable" .)