Provable proposition (7)
I believe the answer is:
theorem
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">provable proposition</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for theorem that I've seen before include "(Maths) proposition" , "Mathematical axiom" , "Proven hypothesis" , "Geometric proposition" , "deduction" .)