Thereby, one understanding nothing must keep quiet mostly (4,5)
I believe the answer is:
ipso facto
'thereby' is the definition.
(Latin phrase meaning thereby or by that very fact)
I cannot understand how the rest of the clue works.
(Other definitions for ipso facto that I've seen before include "Thereby (Latin)" , "By that very act (Lat.)" , "By that truth (Latin)" , "Self-evidently; thereby" , "For that precise reason" .)