Turn into lunatic, eating current food (9)
I believe the answer is:
nutrition
'food' is the definition.
(I know that nutrition is a type of food)
'turn into lunatic eating current' is the wordplay.
'lunatic' indicates anagramming the letters.
'eating' is an insertion indicator.
'current' becomes 'I' (symbol for electric current).
'turn'+'into'='turninto'
'turninto' anagrammed gives 'nutriton'.
'nutriton' placed around 'i' is 'NUTRITION'.
(Other definitions for nutrition that I've seen before include "Food and drink, edibles" , "Food, nourishment" , "form of sustenance" , "Food, its study" , "Act of nourishing" .)