Burn toppled arch (4)
I believe the answer is:
char
'burn' is the definition.
(charring is a kind of burning)
'toppled arch' is the wordplay.
'toppled' indicates anagramming the letters (the order of letters topples or collapses).
'arch' anagrammed gives 'CHAR'.
(Other definitions for char that I've seen before include "Burn slightly" , "toast far too much?" , "'Mrs Mopp'" , "way to make carbon" , "Daily cleaner; burn" .)