Each one a fairy (4)
I believe the answer is:
peri
'a fairy' is the definition.
(Persian mythological being)
'each one' is the wordplay.
'each' becomes 'per' ('per' can mean for each).
'one' becomes 'i' (Roman numeral).
'per'+'i'='PERI'
(Other definitions for peri that I've seen before include "Supernatural being in Persian folklore" , "Spirit of folklore" , "A vision of loveliness" , "Ripe as a kind of fairy" , "Persian fairy or genie" .)