Fruitless tries converted by the French (7)
I believe the answer is:
sterile
'fruitless' is the definition.
(I've seen this before)
'tries converted by the french' is the wordplay.
'converted' indicates an anagram.
'by' is a charade indicator (letters next to each other).
'the french' becomes 'le'.
'tries' anagrammed gives 'steri'.
'steri'+'le'='STERILE'
(Other definitions for sterile that I've seen before include "Free from bacteria" , "Antiseptic" , "Bacteria-free" , "Unproductive" , "Incapable of reproducing" .)