Formal promise (9)
(Other definitions for guarantee that I've seen before include "Formal assurance" , "Underwrite; promise with certainty" , "Provide financial security for" , "Undertake" , "Written assurance, pledge" .)
'formal promise' 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 "Curse or solemn word" , "A curse or solemn declaration" , "darn, for example" , "A word with a witness" , "word intended?" .)