At the piano, he may sound a bit monotonous (5)
I believe the answer is:
tuner
'at the piano he may sound a bit monotonous' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
This is the entire clue.
Can you help me to learn more?
(Other definitions for tuner that I've seen before include "Adjuster of pianos" , "piano man?" , "Radio receiver" , "Motor specialist" , "Radio adjuster" .)