He sings a number of bits of opera (5)
I believe the answer is:
tenor
'he sings a' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'number of bits of opera' is the wordplay.
'number' becomes 'ten' (ten is a number).
'of' becomes 'o''.
'bits of opera' becomes 'r' (I can't justify this - if you can you should give a lot more credence to this answer).
'ten'+'o'+'r'='TENOR'
Can you help me to learn more?
(Other definitions for tenor that I've seen before include "exact words" , "Singer; drift" , "Eg, Placido Domingo" , "character" , "Singer (one of three?)" .)