Gross painting that's rushed inside (6)
I believe the answer is:
arrant
'gross' is the definition.
(I know that gross can be written as arrant)
'painting that's rushed inside' is the wordplay.
'painting' becomes 'art' (painting is an art).
'that's rushed' becomes 'ran'.
'inside' is an insertion indicator.
'art' placed around 'ran' is 'ARRANT'.
(Other definitions for arrant that I've seen before include "Rat ran badly and became unmitigated" , "Out-and-out, downright - often nonsense" , "Utter and extreme, maybe nonsense" , "'Complete, stark (6)'" , "Out-and-out, notorious" .)