Firebug's crime (5)
I believe the answer is:
'firebug's crime' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for arson that I've seen before include "A burning crime" , "Felonious burning of property" , "Crime of fire setting" , "Iniquitous behaviour" , "Crime of fire raising" .)