"Semper Fidelis" composer (5)
I believe the answer is:
sousa
'semper fidelis composer' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for sousa that I've seen before include "Composer of "The Stars and Stripes Forever"" , "US march king" , "March man?" , "he produced the Washington Post" , "American composer known for marches" .)