Mouth organ? (6)
I believe this is a double definition.
'mouth' is the first definition.
'organ?' is the second definition.
(tongue is a kind of organ)
(Other definitions for tongue that I've seen before include "Go tune up what's in the mouth" , "Organ of taste" , "Speech" , "Flap under shoelaces" , "manner of speaking" .)
'mouth organ' is the definition.
This is all the clue.
(Other definitions for harmonica that I've seen before include "An alternative mouth-organ" , "Mouth organ" , "musical instrument" , "Larry Adler's instrument" , "It's blown" .)