Reduce in value (6)
'reduce in value' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for debase that I've seen before include "Degrade or devalue" , "Depreciate or humiliate" , "Lower in quality, reduce the value of" , "Degrade or demean" , "Lower in quality or value, corrupt" .)
(Other definitions for lower that I've seen before include "Let down, being inferior" , "Depress" , "Threaten" , "Inferior - scowl" , "Hereford, say" .)