It's all in your head (5)
I believe the answer is:
brain
'it's all in your head' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for brain that I've seen before include "It's in the skull" , "Intelligent person" , "Encephalon" , "Haemorrhaging of talent" , "Whack on the head" .)