He or I enter set-up for the hypotheses (8)
I believe the answer is:
theories
'the hypotheses' is the definition.
('theory' can be a synonym of 'hypothesis')
'he or i enter set-up' is the wordplay.
'enter' indicates putting letters inside.
'up' says the letters should be written backwards (in down clue: letters go upwards).
'set' backwards is 'tes'.
'he'+'or'+'i'='heori'
'heori' going into 'tes' is 'THEORIES'.
'for' acts as a link.
(Other definitions for theories that I've seen before include "Abstract principles" , "Set of principles on which practices are based" , "premises?" , "General principles or opinions" , "General principles or possibilities" .)