Zealot on charter round island promoting growth (8)
I believe the answer is:
nutrient
'promoting growth' is the definition.
(providing nutrition)
'zealot on charter round island' is the wordplay.
'zealot' becomes 'nut' (both can mean an enthusiast).
'on' is a charade indicator (letters next to each other).
'charter' becomes 'rent' (charter can mean to rent or hire).
'round' indicates putting letters inside.
'island' becomes 'i'.
'rent' going around 'i' is 'rient'.
'nut'+'rient'='NUTRIENT'
(Other definitions for nutrient that I've seen before include "vitamin maybe" , "promoting growth" , "Substance that nourishes" , "Substance essential for growth" , "Item of food" .)