Instrument is unexceptional - no monotonous sound (4)
I believe the answer is:
drum
This could be a double definition.
'instrument' is the first definition.
(I know that drum is a type of percussion instrument)
'unexceptional no monotonous sound' is the second definition.
'drum' can be an answer for 'sound' (drum is a kind of sound). I am not certain of the 'unexceptional no monotonous' bit.
'is' is the link.
Can you help me to learn more?
(Other definitions for drum that I've seen before include "See 16" , "Percussion instrument" , "house" , "one will be beaten" , "Empty" .)