When one might eat lavishly, but only after fasting initially (6)
I believe the answer is:
easter
'might eat lavishly but only after fasting initially' is the definition.
I can't judge whether this definition defines the answer.
'when one' is the wordplay.
'when' becomes 'e' (I've seen this in other clues).
'one' becomes 'aster' (I can't justify this - if you can you should believe this answer much more).
'e'+'aster'='EASTER'
Can you help me to learn more?
(Other definitions for easter that I've seen before include "Parade; egg; lily" , "A terse sort of Spring festival?" , "Important Christian festival" , "break" , "Church festival" .)