Wicked thing with fire in grates, not initially working (5)
I believe the answer is:
arson
'wicked thing with fire' is the definition.
(fire-based crime)
'grates not initially working' is the wordplay.
'grates' becomes 'jars' (I've seen this before).
'not initially' means to remove the first letter.
'working' becomes 'on' (e.g. a machine is working or on).
'jars' with its initial letter taken off is 'ars'.
'ars'+'on'='ARSON'
'in' acts as a link.
(Other definitions for arson that I've seen before include "Fire-lighting crime" , "The act of firing" , "serious offence" , "Torching" , "Fire-raiser's crime" .)