Conjecture, hypothesis (6)
I believe the answer is:
theory
This clue is a double definition.
'conjecture' is the first definition.
(I've seen this before)
'hypothesis' is the second definition.
(I know that hypothesis can be written as theory)
(Other definitions for theory that I've seen before include "Set of ideas formulated to explain something" , "Unproven explanation" , "abstract reasoning" , "Idea formulated to explain something" , "Hypothesis or concept" .)