Bear witness in appointment set tactfully up (6)
I believe the answer is:
attest
'bear witness' is the definition.
('attest' can be a synonym of 'bear witness')
'in appointment set tactfully up' is the wordplay.
'in' indicates a hidden word.
'up' is a reversal indicator (in down clue: letters go upwards).
'tsetta' is hidden in the letters of 'appointment set tactfully'.
'tsetta' in reverse letter order is 'ATTEST'.
(Other definitions for attest that I've seen before include "Provide proof of, declare to be true (6)" , "Act as witness" , "Bear witness, declare to be true" , "Give proof" , "Bear formal witness to, give evidence" .)