Rascal disappointed with burn (5)
I believe the answer is:
scald
'burn' is the definition.
(scald is a kind of burn)
'rascal disappointed with' is the wordplay.
'with' indicates the answer is hidden within the clue.
'SCALD' is hidden in the letters of 'rascal disappointed'.
(Other definitions for scald that I've seen before include "Injure with steam" , "Burn with water" , "Burn with hot liquid" , "Injure with hot liquid" , "Heat (milk) to near boiling point" .)