Appear in court, perhaps, where proceedings can last up to five days (6)
I believe the answer is:
attest
'appear' is the definition.
Although both the answer and definition are verbs in their base form, I can't understand how one could define the other.
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for attest that I've seen before include "Demonstrate" , "Give evidence - sounds a trial" , "Swear to" , "Give proof" , "Bear formal witness to, give evidence" .)