Accepted by party members, he provides hypotheses (8)
I believe the answer is:
theories
'hypotheses' is the definition.
('theory' can be a synonym of 'hypothesis')
'accepted by party members he' is the wordplay.
'accepted by' means one lot of letters goes inside another (inserted letters are accepted).
'party members' becomes 'tories' (I can't explain this - if you can you should believe this answer much more).
'tories' going around 'he' is 'THEORIES'.
'provides' acts as a link.
Can you help me to learn more?
(Other definitions for theories that I've seen before include "Assumptions" , "General principles or opinions" , "Conjectures" , "Abstract principles" , "Explanatory suppositions" .)