Calculus pioneer (5)
I believe the answer is:
euler
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">calculus pioneer</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for euler that I've seen before include "calculating foreigner" , "Swiss mathematician; sounds like lubricator" , "See fifty three across" , "Leonhard -, C-eighteen Swiss mathematician" , "Eighteen-hole game" .)