Evidence in court set up — Athenian guilty finally (9)
I believe the answer is:
testimony
'evidence in court' is the definition.
(a witness may give testimony in court)
'set up athenian guilty finally' is the wordplay.
'up' is a reversal indicator (in down clue: letters go upwards).
'athenian' becomes 'timon' (as in Shakespeare's Timon of Athens).
'finally' says to take the final letters.
The last letter of 'guilty' is 'y'.
'set' in reverse letter order is 'tes'.
'tes'+'timon'+'y'='TESTIMONY'
(Other definitions for testimony that I've seen before include "It's given under oath" , "Court statement" , "Evidence, declaration" , "witness statement" , "Evidence offered by a witness" .)