Food fan one in charge (8)
I believe the answer is:
nutrient
'food' is the definition.
(synonyms)
'fan one in charge' is the wordplay.
'fan' becomes 'nut' (both can mean an enthusiast).
'one' becomes 'i' (Roman numeral).
'in' indicates putting letters inside.
'charge' becomes 'rent' (I've seen this in other clues).
'i' placed within 'rent' is 'rient'.
'nut'+'rient'='NUTRIENT'
(Other definitions for nutrient that I've seen before include "Substance providing nourishment" , "Substance essential for growth" , "promoting growth" , "Nourishing substance" , "Good food" .)