Wouldn't he sing for a fiver? (5)
I believe the answer is:
tenor
'wouldn't he sing for a fiver?' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
This is all the clue.
Can you help me to learn more?
(Other definitions for tenor that I've seen before include "Vocal range" , "Highest male voice" , "intent" , "Male 17" , "Male voice of high pitch, like Luciano" .)