Mason is put out making conjecture (10)
I believe the answer is:
assumption
'conjecture' is the definition.
(thesaurus)
'mason is put out' is the wordplay.
'out' indicates an anagram (out can mean wrong or inaccurate).
'mason'+'is'+'put'='masonisput'
'masonisput' is an anagram of 'ASSUMPTION'.
'making' acts as a link.
(Other definitions for assumption that I've seen before include "It's supposed" , "Summer feast" , "Supposition" , "Catholic feast" , "Church festival" .)