What the witness gives evidence on (4)
I believe the answer is:
oath
'evidence on' is the definition.
The answer and definition can be both related to communication as well as being singular nouns.
Perhaps there's an association between them I don't understand?
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for oath that I've seen before include "Solemn pledge" , "Vow; profanity" , "Vow, sworn word" , "Curse or solemn word" , "Imprecation" .)