Pioneer in probability (6)
I believe the answer is:
'pioneer in probability' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for pascal that I've seen before include "Unit of pressure, French philosopher and programming language" , "French mathematician, d. 1662 - SI unit of pressure" , "17th century French mathematician -- SI unit of pressure" , "Unit of pressure; French scientist" , "Seventeenth century French physicist" .)