He composed in subtle harmonies (5)
I believe the answer is:
lehar
'he composed' is the definition.
The definition suggests a singular noun which matches the answer.
'in subtle harmonies' is the wordplay.
'in' indicates a hidden word.
'LEHAR' is hidden within 'subtle harmonies'.
(Other definitions for lehar that I've seen before include "Operetta composer" , "He scored" , "Operettist" , "Austrian of note" , "Franz . . . . . composed 'The Merry Widow'" .)