On French island, fail to see weapon (7)
I believe the answer is:
missile
'weapon' is the definition.
(I know that missile is a type of weapon)
'on french island fail' is the wordplay.
'on' means one lot of letters go next to another.
'french island' becomes 'ile' ('island' in French).
'fail' becomes 'miss' (missing is a kind of failing).
'ile' put after 'miss' is 'MISSILE'.
'to see' acts as a link.
(Other definitions for missile that I've seen before include "Weapon that's thrown or projected" , "weapon that's fired" , "One could get fired" , "Weapon that is propelled" , "Projectile (sometimes guided)" .)