Part of a swearing-in ceremony (4)
I believe the answer is:
oath
'part of a swearing-in ceremony' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for oath that I've seen before include "Swear word" , "What frustrated people say" , "A curse or a vow" , "Swearword or solemn word" , "One sworn" .)