A number possibly hear operettas he composed (5)
I believe the answer is:
lehar
'operettas he composed' is the definition.
The definition suggests a singular noun which matches the answer.
'a number possibly hear' is the wordplay.
'a number' becomes 'l' (fifty in Roman numerals).
'possibly' indicates anagramming the letters.
'hear' with letters rearranged gives 'ehar'.
'l'+'ehar'='LEHAR'
Can you help me to learn more?
(Other definitions for lehar that I've seen before include "Franz -----, composer of The Merry Widow" , "Operetta composer" , "Musician" , "Franz . . . . . composed 'The Merry Widow'" , "Operettist" .)