First of all, euphonium and reed organ (3)
I believe the answer is:
ear
'organ' is the definition.
(the ear is an organ)
'first of all euphonium and reed' is the wordplay.
'first of all' indicates taking the first letters.
The initial letters of 'euphonium and reed' is 'EAR'.
(Other definitions for ear that I've seen before include "cut" , "Spike (wheat)" , "That makes sense" , "Bit of corn" , "'Give every man thine ..., but few thy voice'/Hamlet" .)