It's more than a hypothesis (6)
I believe the answer is:
theory
'it's more than a hypothesis' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for theory that I've seen before include "Unproven explanation" , "Guess" , "General principles of a subject" , "Hypothesis, thesis" , "Supposition, set of ideas" .)