Lead-in to "calculus" (3)
I believe the answer is:
pre
'lead-in to calculus' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for pre that I've seen before include "Before starting" , "Before or ahead of" , "Combining form meaning before" , "Before, prior to" , "Prefix meaning before" .)