It's not true I'm over obsession (6)
I believe the answer is:
mirage
'it's not true' is the definition.
(a mirage is an illusory image)
'i'm over obsession' is the wordplay.
'over' says the letters should be written in reverse.
'obsession' becomes 'rage' (rage can mean an obsession or trend).
'im' back-to-front is 'mi'.
'mi'+'rage'='MIRAGE'
(Other definitions for mirage that I've seen before include "Something illusory and unattainable" , "fantasy" , "French fighter aircraft" , "Hallucination caused by hot air" , "Optical illusion, in desert perhaps" .)