Food fanatic keeping fit, even empty (9)
I believe the answer is:
nutriment
'food' is the definition.
(nutriment is a kind of food)
'fanatic keeping fit even empty' is the wordplay.
'fanatic' becomes 'nut' (I've seen this before).
'keeping' means one lot of letters goes inside another.
'fit' becomes 'trim' ('trim' can be a synonym of 'fit'**).
'empty' suggests removing the centre.
'even' with its centre taken out is 'en'.
'trim'+'en'='trimen'
'nut' placed around 'trimen' is 'NUTRIMENT'.
(Other definitions for nutriment that I've seen before include "diet" , "Sustenance, nourishment" , "Nourishing food" , "It sustains" , "Material providing food" .)