Thirteen Enigma Variations Elgar originally pictured within Nimrod puzzled composer (10)
I believe the answer is:
monteverdi
'composer' is the definition.
(Claudio Monteverdi, 1567-1643)
'thirteen enigma variations elgar originally pictured within nimrod puzzled' is the wordplay.
'originally' suggests taking the first letters.
'pictured within' indicates putting letters inside.
'puzzled' indicates an anagram.
The initial letters of 'thirteen enigma variations elgar' is 'teve'.
'nimrod' is an anagram of 'monrdi'.
'teve' put inside 'monrdi' is 'MONTEVERDI'.
(Other definitions for monteverdi that I've seen before include "Claudio -, an early composer of operas" , "Composer of "Orpheus"" , "Claudio --, 16th-17th century Italian composer" , "Italian scorer" , "One composer" .)