One French conclusion is interminable (8)
I believe the answer is:
unending
'interminable' is the definition.
(I've seen this before)
'one french conclusion' is the wordplay.
'one french' becomes 'un'.
'conclusion' becomes 'ending' (synonyms).
'un'+'ending'='UNENDING'
'is' acts as a link.
(Other definitions for unending that I've seen before include "Countless; continual" , "Constant" , "Continuing indefinitely" , "Everlasting" , "Limitless" .)