Element of initiation ceremony in days gone by (4)
I believe the answer is:
once
'in days gone by' is the definition.
(I've seen this before)
'element of initiation ceremony' is the wordplay.
'element of' says the answer is hidden in the clue (I've seen this in other clues).
'ONCE' is hidden within 'initiation ceremony'.
(Other definitions for once that I've seen before include "Long ago" , "In times past" , "Quick examination" , "Formerly - not repeated" , "in days of yore?" .)