Among cunning old people there’s one mathematician (10)
I believe the answer is:
archimedes
'mathematician' is the definition.
(I've seen this before)
'among cunning old people there's one' is the wordplay.
'among' is an insertion indicator (I've seen this in other clues).
'cunning' becomes 'arch' (I've seen this in other clues).
'old people' becomes 'medes' (people of the Median Empire).
'there's one' becomes 'i' (I can't explain this - if you can you should give a lot more credence to this answer).
'arch'+'medes'='archmedes'
'archmedes' placed around 'i' is 'ARCHIMEDES'.
(Other definitions for archimedes that I've seen before include "Ancient Greek mathematician/inventor" , "Greek physicist" , "Ancient Greek scientist (of buoyancy principle)" , "Syracusan" , "Greek mathematician ('Eureka')" .)