Most calm, and yet is French! (8)
I believe the answer is:
stillest
'most calm' is the definition.
('still' can be similar in meaning to 'calm')
'yet is french' is the wordplay.
'yet' becomes 'still' (synonyms).
'is french' becomes 'est' ('is' in French).
'still'+'est'='STILLEST'
'and' acts as a link.
(Other definitions for stillest that I've seen before include "least enterprising" , "Most calm" , "Least disruptive" .)