Online nuisance, pest finally taking tumble (5)
I believe the answer is:
troll
'online nuisance' is the definition.
(troll can mean an annoying person online)
'pest finally taking tumble' is the wordplay.
'finally' indicates one should take the final letters.
'taking' means one lot of letters go next to another.
'tumble' becomes 'roll' (tumble can mean to fall or roll).
The final letter of 'pest' is 't'.
't'+'roll'='TROLL'
(Other definitions for troll that I've seen before include "Supernatural cave-dweller" , "who's stirring on-line" , "Monster - fish" , "Cave-dwelling dwarf or giant" , "unwanted contributor to site?" .)