Time to crush online troublemaker (5)
I believe the answer is:
troll
'online troublemaker' is the definition.
(abusive or annoying person online)
'time to crush' is the wordplay.
'time' becomes 't' (abbreviation).
'to crush' becomes 'roll' (both can mean to press down on).
't'+'roll'='TROLL'
(Other definitions for troll that I've seen before include "Ugly Norse dwarf" , "Bad fairy (giant or dwarf)" , "Ugly dwarf or giant" , "Malignant elf" , "Supernatural cave-dweller" .)