People in general accept alternative supposition (6)
I believe the answer is:
theory
'supposition' is the definition.
(thesaurus)
'people in general accept alternative' is the wordplay.
'people in general' becomes 'they' (I've seen this before).
'accept' means one lot of letters goes inside another.
'alternative' becomes 'or'.
'they' placed around 'or' is 'THEORY'.
(Other definitions for theory that I've seen before include "Conjecture, hypothesis" , "Idea to explain something, say evolution" , "Speculative view" , "Conjectural view" , "Set of ideas formed from reasoning" .)