Those people hold alternative hypothesis (6)
I believe the answer is:
theory
'hypothesis' is the definition.
(I know that hypothesis can be written as theory)
'those people hold alternative' is the wordplay.
'those people' becomes 'they' (I've seen this in other clues).
'hold' is an insertion indicator.
'alternative' becomes 'or'.
'they' placed around 'or' is 'THEORY'.
(Other definitions for theory that I've seen before include "Proposed explanation of something" , "Explanatory idea" , "Abstract explanation" , "General principle which explains some aspect of the world" , "Scientific conjecture" .)