Small son retaining very French accent (6)
I believe the answer is:
stress
'accent' is the definition.
(synonyms)
'small son retaining very french' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'son' becomes 's'.
'retaining' means one lot of letters goes inside another.
'very french' becomes 'tres' ('very' translated into French).
's'+'s'='ss'
'ss' placed around 'tres' is 'STRESS'.
(Other definitions for stress that I've seen before include "Strain or accentuate" , "Cause of crack" , "Force" , "Tension, pressure" , "possible excuse for sickie?" .)