Explosive one contained by fat old polymath (8)
I believe the answer is:
leonardo
'polymath' is the definition.
(I know that Leonardo has the occupation polymath)
'explosive one contained by fat old' is the wordplay.
'explosive' indicates an anagram.
'contained by' indicates putting letters inside.
'fat' becomes 'lard' (lard is a kind of fat).
'old' becomes 'o' (abbreviation).
'one' is an anagram of 'eon'.
'eon' put inside 'lard' is 'leonard'.
'leonard'+'o'='LEONARDO'
(Other definitions for leonardo that I've seen before include "Great Italian artist ........ da Vinci" , "US actor, - DiCaprio" , "and 12 Across: Renaissance genius" , "Great painter of the Mona Lisa, . . . . . . . . da Vinci" , "Da Vinci and DiCaprio" .)