Expert's against French art shows (6)
I believe the answer is:
proves
'shows' is the definition.
('prove' can be a synonym of 'show')
'expert's against french art' is the wordplay.
'expert' becomes 'pro' (I've seen this before).
'against' becomes 'v' (abbreviation for 'versus').
'french art' becomes 'es' ('is' in French - 'art' can mean 'is' eg 'thou art').
'pro'+'v'+'es'='PROVES'
(Other definitions for proves that I've seen before include "Verifies" , "Provides evidence for" .)