Degradation of French cellar (10)
I believe the answer is:
debasement
'degradation' is the definition.
(I know that degradation can be written as debasement)
'of french cellar' is the wordplay.
'of french' becomes 'de' ('of' in French).
'cellar' becomes 'basement' (both can mean the space underneath a house).
'de'+'basement'='DEBASEMENT'
(Other definitions for debasement that I've seen before include "Degradation" , "What's lowering" , "Dishonour" .)