To keep the French in sight requires a certain skill (7)
I believe the answer is:
sleight
'a certain skill' is the definition.
'sleight' can be an answer for 'skill' (I have seen 'Handy skill' mean 'sleight' so perhaps 'skill' could also mean 'sleight'). I am unsure of the 'a certain' bit.
'to keep the french in sight' is the wordplay.
'to keep the french' becomes 'le' (I can't justify this - if you can you should believe this answer much more).
'in' means one lot of letters goes inside another.
'le' inserted within 'sight' is 'SLEIGHT'.
'requires' acts as a link.
Can you help me to learn more?
(Other definitions for sleight that I've seen before include "craft" , "Conjuror uses this" , "It's deceiving to the eye" , "Tricky cunning" , "Handy skill" .)