Was he with gross cover-up? (9)
I believe the answer is:
whitewash
'cover-up?' is the definition.
(I've seen this in another clue)
'was he with gross' is the wordplay.
'gross' indicates an anagram.
'was'+'he'+'with'='washewith'
'washewith' is an anagram of 'WHITEWASH'.
(Other definitions for whitewash that I've seen before include "Cover up - wall coating" , "Substance applied to a wall - total defeat" , "Gloss over faults with wall colouring?" , "Solution for painting walls - clean sweep of victories" , "overwhelming defeat" .)