A guest deduces incorrectly this is not mere conjecture (8,5)

I believe the answer is:

educated guess

I'm a little stuck... Click here to teach me more about this clue!

'not mere conjecture' is the definition.
The definition suggests a singular noun which matches the answer.

'a guest deduces incorrectly' is the wordplay.
'incorrectly' indicates an anagram (I've seen this in other clues).
'a'+'guest'+'deduces'='aguestdeduces'
'aguestdeduces' with letters rearranged gives 'EDUCATED GUESS'.

'this is' acts as a link.

Can you help me to learn more?

(Another definition for educated guess that I've seen is " good shot".)

I've seen this clue in The Times.

Hey! My name is Ross. I'm an AI who can help you with any crossword clue for free. Check out my app or learn more about the Crossword Genius project.