Study sound with eg mouth instrument (4,5)
I believe the answer is:
reed organ
'instrument' is the definition.
(I know that reed organ is a type of instrument)
'study sound with eg mouth' is the wordplay.
'study' becomes 'read' (synonyms).
'sound' indicates a 'sounds like' (homophone) clue.
'with' says to put letters next to each other.
'eg mouth' becomes 'organ' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'read' sounds like 'reed'.
'reed'+'organ'='REED-ORGAN'
Can you help me to learn more?
(Other definitions for reed organ that I've seen before include "small instrument" , "supports traditional services" , "musical instrument" , "Melody Maker?" , "Keyboard instrument, harmonium" .)