Instruction for the French boy (6)
I believe the answer is:
'instruction' is the definition.
(I know that lesson is a type of instruction)
'the french boy' is the wordplay.
'the french' becomes 'les' (plural form of 'the' in French).
'boy' becomes 'son' (synonyms).
'for' acts as a link.
(Other definitions for lesson that I've seen before include "15 down period" , "Class session or reading from Bible" , "Teaching period" , "Single period of instruction" , "warning message" .)