Old King that is taking on good composer (5)
I believe the answer is:
grieg
'composer' is the definition.
(Norwegian composer Edvard Grieg 1843-1907)
'old king that is taking on good' is the wordplay.
'old king' becomes 'gr' (Georgius Rex, Latin for King George).
'that is' becomes 'ie' ('i.e.' can be a synonym of 'that is').
'taking on' means one lot of letters go next to another.
'good' becomes 'g' (abbreviation).
'gr'+'ie'+'g'='GRIEG'
(Other definitions for grieg that I've seen before include "Composer of incidental music to Peer Gynt, d.1907" , "Edvard ---, composer" , "Composer of the incidental music for Ibsen's Peer Gynt" , "Scandinavian composer, d. 1907" , "Norwegian composer (Peer Gynt)" .)