Cover up heavy defeat (9)
I believe the answer is:
whitewash
I believe this is a double definition.
'cover up' is the first definition.
(synonyms)
'heavy defeat' is the second definition.
(a whitewash is a serious defeat)
(Other definitions for whitewash that I've seen before include "Used on walls to cover up a misdemeanour?" , "Gloss over faults with wall colouring?" , "Solution for painting walls - clean sweep of victories" , "Gloss over mistakes or faults, using wall or celing paint? (9)" , "overwhelming defeat" .)