Rituals the French introduced, serving no purpose (7)
I believe the answer is:
useless
'serving no purpose' is the definition.
(having no purpose or function)
'rituals the french introduced' is the wordplay.
'rituals' becomes 'uses' (the religious ritual of a particular diocese).
'the french' becomes 'les' (plural form of 'the' in French).
'introduced' indicates putting letters inside.
'uses' enclosing 'les' is 'USELESS'.
(Other definitions for useless that I've seen before include "Vain - unavailing" , "Serving no practical purpose" , "Counterproductive" , "Unserviceable" , "Incompetent" .)