Sloppy presenter hypes a single speculative assumption (10)
I believe the answer is:
hypothesis
'assumption' is the definition.
(thesaurus)
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for hypothesis that I've seen before include "conjecture" , "What's supposed" , "Tentative proposition" , "Proposed explanation" , "Guess" .)