The March King (5)
I believe the answer is:
sousa
'the march king' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for sousa that I've seen before include "he produced the Washington Post" , "US composer of marches" , "John Philip --, American march composer" , "John Philip ..., composer of military marches" , "John Philip ---, US bandmaster" .)