Almost understand Frenchman being calm (6)
I believe the answer is:
serene
'being calm' is the definition.
('serene' can be a synonym of 'calm')
'almost understand frenchman' is the wordplay.
'almost' means to remove the last letter (most of the word but not all of it).
'understand' becomes 'see' (synonyms).
'frenchman' becomes 'rene' (common French name).
'see' with its last letter removed is 'se'.
'se'+'rene'='SERENE'
(Other definitions for serene that I've seen before include "Calm and unruffled, like a princely title." , "Peaceful and unruffled" , "Placid" , "Calm, untroubled" , "still" .)