Once upon a time, one of great skill (6)
I believe the answer is:
expert
'one of great skill' is the definition.
Both the answer and definition are adjectives. Maybe they are linked in a way I don't understand?
'once upon a time' is the wordplay.
'once' becomes 'ex' (prefix meaning former or onetime).
'upon' is a charade indicator (letters next to each other).
'a' becomes 'per' (eg 'one a day' means 'one per day').
'time' becomes 't' (abbreviation).
'ex'+'per'+'t'='EXPERT'
Can you help me to learn more?
(Other definitions for expert that I've seen before include "One practised" , "Pro" , "Master" , "Highly skilled or knowledgeable person" , "Person with great knowledge or skill, specialist" .)