Tedious task; one dragged on (3)
I believe the answer is:
fag
'tedious task' is the definition.
'fag' can be an answer for 'task' (I have seen 'Boring task' mean 'fag' so perhaps 'task' could also mean 'fag'). I'm unsure of the 'tedious' bit.
'one dragged on' is the wordplay.
'one' becomes 'a' (a thing is one thing).
'dragged' becomes 'fg' (I can't justify this - if you can you should believe this answer much more).
'on' means one lot of letters goes inside another (as in clothing 'on' a person).
'a' going into 'fg' is 'FAG'.
Can you help me to learn more?
(Other definitions for fag that I've seen before include "Boring task" , "Cigarette for a menial schoolboy" , "Tedious chore" , "smoke" , "He'll do anything you ask" .)