Supposition (6)
'supposition' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for theory that I've seen before include "General principles of a subject" , "Series of ideas seeking to explain something" , "can provide an explanation" , "Conjectural view" , "Explanatory idea" .)
'supposition' is the definition.
(synonyms)
This is all the clue.
(Other definitions for conjecture that I've seen before include "Unproven opinion or conclusion" , "Unproven theorem" , "Supposition" , "Something supposed" , "Fancy" .)