Awful mistake, supporting Polish (7)
I believe the answer is:
rubbish
'awful' is the definition.
(I've seen this before)
'mistake supporting polish' is the wordplay.
'mistake' becomes 'bish' (I've seen this before).
'supporting' says to put letters next to each other (in a down clue, some letters go below others).
'polish' becomes 'rub' (polish meaning rub to make shiny).
'bish' put after 'rub' is 'RUBBISH'.
(Other definitions for rubbish that I've seen before include "Worthless material to discard" , "Stuff and nonsense" , "Refuse, junk" , "Dross - twaddle" , "make a scathing comment" .)