Take part in initiation ceremony at any time (4)
I believe the answer is:
once
'at any time' is the definition.
(thesaurus)
'take part in initiation ceremony' is the wordplay.
'take part in' indicates the answer is hidden within the clue.
'ONCE' can be found hidden inside 'initiation ceremony'.
(Other definitions for once that I've seen before include "Formerly; as soon as" , "A single time" , "Previously" , "Some time back" , "Quick examination" .)