Throw mud at grenade -- it goes off (9)
I believe the answer is:
denigrate
'throw mud at' is the definition.
(both can mean to criticise or disparage)
'grenade it goes off' is the wordplay.
'goes off' indicates anagramming the letters.
'grenade'+'it'='grenadeit'
'grenadeit' anagrammed gives 'DENIGRATE'.
(Other definitions for denigrate that I've seen before include "put down" , "Run down" , "Defame" , "Unfairly criticise" , "Criticise, belittle" .)