This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
訯 combines 言 (speech) with 及 (reach/extend). The exact historical development is uncertain, but it likely relates to speech or communication.
With 言 (speech) and 及 (reach), imagine words reaching out to chatter or talk, but the exact meaning is unclear.
For サ, picture someone saying 'sa' as they start to speak: the sound 'sa' cues サ, and the speech radical 言 reinforces the idea of talking.