Son, very French, beginning to suffer tension (6)
I believe the answer is:
stress
'tension' is the definition.
('stress' can be a synonym of 'tension')
'son very french beginning to suffer' is the wordplay.
'son' becomes 's'.
'very french' becomes 'tres' ('very' translated into French).
'beginning to' suggests taking the first letters.
The first letter of 'suffer' is 's'.
's'+'tres'+'s'='STRESS'
(Other definitions for stress that I've seen before include "why off work?" , "Particular emphasis" , "nervous tension" , "Emphasis on anxiety?" , "Emotional tension" .)