Second son retains very French accent (6)
I believe the answer is:
stress
'accent' is the definition.
(synonyms)
'second son retains very french' is the wordplay.
'second' becomes 's' (common abbreviation).
'son' becomes 's' (genealogical abbreviation for son).
'retains' is an insertion indicator.
'very french' becomes 'tres' ('very' in French).
's'+'s'='ss'
'ss' placed around 'tres' is 'STRESS'.
(Other definitions for stress that I've seen before include "Emphasise the pressure" , "Force" , "Ictus" , "Draw attention to" , "possible cause of fracture?" .)