Small son retains very French accent (6)
I believe the answer is:
stress
'accent' is the definition.
(I know that accent can be written as stress)
'small son retains very french' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'son' becomes 's' (genealogical abbreviation for son).
'retains' is an insertion indicator.
'very french' becomes 'tres' ('very' in French).
's'+'s'='ss'
'ss' going around 'tres' is 'STRESS'.
(Other definitions for stress that I've seen before include "Tension, pressure" , "Feeling of pressure" , "Emphasise the strain" , "State mental or emotional strain" , "Underline - tension" .)