Age concealed by fat old artist (8)
I believe the answer is:
leonardo
'artist' is the definition.
(Leonardo is an example)
'age concealed by fat old' is the wordplay.
'age' becomes 'eon' (eon is a kind of age).
'concealed by' indicates putting letters inside.
'fat' becomes 'lard' (I've seen this before).
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'eon' put inside 'lard' is 'leonard'.
'leonard'+'o'='LEONARDO'
(Other definitions for leonardo that I've seen before include "Great Italian artist ........ da Vinci" , "-- di Caprio, actor" , "and 12 Across: Renaissance genius" , "First name of da Vinci, great artist" , "Da Vinci's name" .)