Intrinsic nature (7)
I believe the answer is:
essence
'intrinsic nature' is the definition.
(essence can mean something's nature or core)
This is all the clue.
(Other definitions for essence that I've seen before include "required property" , "Flavour, spirit" , "Perfume or concentrate" , "SE scene (anag.)" , "Extract; intrinsic nature" .)