Mathematician once corresponded in a matter of law (10)
I believe the answer is:
archimedes
'mathematician once' is the definition.
(ancient Greek mathematician)
'corresponded in a matter of law' is the wordplay.
'corresponded' becomes 'chimed' (chime can mean to correspond or match).
'in' means one lot of letters goes inside another.
'matter of law' becomes 'res' (legal term for a thing).
'a'+'res'='ares'
'chimed' placed within 'ares' is 'ARCHIMEDES'.
(Other definitions for archimedes that I've seen before include "Old scientist" , "Ancient Greek scientist (of buoyancy principle)" , "Greek mathematician ('Eureka')" , "Old mathematician" , "Ancient Greek mathematician/inventor" .)