Some oaths we are to … this (5)
I believe the answer is:
swear
'this' is the definition.
The definition and answer are not the same part of speech.
'some oaths we are' is the wordplay.
'some' indicates a hidden word (some of the parts of the word are used).
'SWEAR' is hidden in the letters of 'oaths we are'.
'to' acts as a link.
(Other definitions for swear that I've seen before include "Affirm" , "Asseverate" , "Use bad language" , "Solemnly promise" , "Assert" .)