New rag improvised by a composer (6)
I believe the answer is:
Here is my best explanation:
'a composer' is the definition.
(Wagner is an example)
'new rag improvised' is the wordplay.
'improvised' indicates an anagram (I've seen this in other clues).
'newrag' anagrammed gives 'WAGNER'.
'by' acts as a link.
(Other definitions for wagner that I've seen before include "German cycle-maker", "Richard . . . . . . composer of 'The Flying Dutchman' and 'Lohengrin'", "Composer honoured in Bayreuth", "Composer of ''The Flying Dutchman'' etc", "Tannhauser composer", "Composer of Tannhauser", "German composer of The Flying Dutchman", "Noted German", "German opera composer" and "He composed 'The Flying Dutchman'".)