Age concealed by fat old polymath (8)
I believe the answer is:
leonardo
'polymath' is the definition.
(I know that Leonardo has the occupation polymath)
'age concealed by fat old' is the wordplay.
'age' becomes 'eon' (eon is a kind of age).
'concealed by' is an insertion indicator.
'fat' becomes 'lard' (lard is a kind of fat).
'old' becomes 'o' (abbreviation).
'eon' going inside 'lard' is 'leonard'.
'leonard'+'o'='LEONARDO'
(Other definitions for leonardo that I've seen before include "Great painter of the Mona Lisa, . . . . . . . . da Vinci" , "So-called code" , "Painter of three ac" , "Renaissance man" , "Da Vinci and Di Caprio" .)