Vow under oath (5)
I believe the answer is:
swear
'vow under oath' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for swear that I've seen before include "Use bad language" , "Solemnly promise" , "Asseverate" , "Vow, attest" , "Vow; blaspheme" .)