Lethal of French lady's new fashion (6)
I believe the answer is:
deadly
'lethal' is the definition.
(I know that lethal can be written as deadly)
'of french lady's new fashion' is the wordplay.
'of french' becomes 'de' ('of' in French).
'new fashion' is an anagram indicator.
'lady' anagrammed gives 'adly'.
'de'+'adly'='DEADLY'
(Other definitions for deadly that I've seen before include "Lethal, capable of killing" , "Lethal, capable of causing fatality" , "Lethal, capable of causing fatal injury" , "No small bore" , "Belladonna" .)