A black loaf is burning (6)
I believe the answer is:
ablaze
'is burning' is the definition.
(I've seen this before)
'a black loaf' is the wordplay.
'black' becomes 'b' (as in pencil classifications).
'loaf' becomes 'laze' (as in to loaf or laze around idly).
'a'+'b'+'laze'='ABLAZE'
(Other definitions for ablaze that I've seen before include "Flaming; on fire" , "Having been fired" , "On fire, burning" , "being toasted?" , "Well-lit" .)