Mathematician of 81 died (6)
I believe the answer is:
euclid
'mathematician' is the definition.
(Euclid is an example)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for euclid that I've seen before include "Early Greek geometrician" , "Old Greek master of geometry" , "Geometrician of the 3rd century BC" , "6 mathematician" , "Ancient Greek master of geometry" .)