Seabed wrecks reduce in quality (6)
I believe the answer is:
debase
'reduce in quality' is the definition.
(I've seen this before)
'seabed wrecks' is the wordplay.
'wrecks' indicates anagramming the letters.
'seabed' with letters rearranged gives 'DEBASE'.
(Other definitions for debase that I've seen before include "Lower in value or quality, sometimes currency" , "Lower in quality, reduce the value of" , "Humble" , "Degrade or devalue" , "Lower in quality or value, corrupt" .)