Observe musical sound (4)
I believe the answer is:
note
This might be a double definition.
'observe' is the first definition.
(I know that observe can be written as note)
'musical sound' is the second definition.
'note' can be an answer for 'sound' (thesaurus). I'm not certain of the 'musical' bit.
Can you help me to learn more?
(Other definitions for note that I've seen before include "Brief letter - distinction" , "Record - piece of music" , "Pay close attention to" , "Crotchet or minim, say?" , "Eminence" .)