Sworn declaration (4)
I believe the answer is:
oath
'sworn declaration' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for oath that I've seen before include "Solemn vow" , "Swear word" , "A curse or a vow" , "Solemn promise to tell the truth or be loyal" , "Solemn pledge" .)