Promise utter obscenities (5)
I believe the answer is:
swear
'promise' is the definition.
(as in swearing something is true)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for swear that I've seen before include "Assert" , "Asseverate" , "Promise on oath" , "Use bad language" , "Avow" .)