Accept as true without proof (6)
I believe the answer is:
assume
'accept as true without proof' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for assume that I've seen before include "Adopt falsely" , "Adopt or take for granted" , "Suppose; put on" , "Take as true or take for granted" , "Take as true without proof" .)