Lay down as fact (5)
I believe the answer is:
posit
'lay down as fact' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for posit that I've seen before include "Put forward; place" , "Put forward (a theory)" , "state" , "Put forward as true" , "Put forward (something) as a given" .)