Battered bangers turn over hoarding where flag is (4-2-3-4)
I believe the answer is:
toad-in-the-hole
'battered bangers' is the definition.
(dish of battered sausages)
'turn over hoarding where flag is' is the wordplay.
'turn over' becomes 'to' (t.o. can mean 'turn over' on a written document).
'hoarding' becomes 'ad' (a hoarding is a board for advertisements).
'where flag is' becomes 'in the hole' (as in flags on a golf course).
'to'+'ad'+'inthehole'='TOAD-IN-THE-HOLE'
(Other definitions for toad-in-the-hole that I've seen before include "Battered sausage" , "Batter pudding containing sausages" , "Bangers and batter" , "Sausage and batter dish" , "Sausages and batter dish" .)