Lawful support - setback for the French (5)
I believe the answer is:
legal
'lawful' is the definition.
(associated in meaning)
'support setback for the french' is the wordplay.
'support' becomes 'leg' (leg is a kind of support).
'setback for' shows that the letters should be reversed in order (letters are set down in backwards order).
'the french' becomes 'la' (one of the French words for 'the').
'la' reversed gives 'al'.
'leg'+'al'='LEGAL'
(Other definitions for legal that I've seen before include "Constitutional" , "Observing acts" , "not banned" , "Above board" , "In accordance with statute" .)