2004 Britney Spears hit (5)
I believe the answer is:
toxic
'2004 britney spears hit' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for toxic that I've seen before include "Poisonous, noxious" , "word describing bad things" , "Harmful - cox it (anag)" , "Deadly - venomous" .)