Blacken on a barbecue (4)
I believe the answer is:
char
'blacken on a barbecue' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for char that I've seen before include "House cleaner" , "Singe; a fish" , "Partly burn - cleaning woman" , "Clean < fish" , "Scorch: trout-like fish" .)