Stick with left internally disheartened (4)
I believe the answer is:
glum
'disheartened' is the definition.
(both can mean sad)
'stick with left internally' is the wordplay.
'stick with' becomes 'gum' (I've seen this before. I am not sure about the 'with' bit.).
'left' becomes 'L' (common abbreviation).
'internally' is an insertion indicator.
'gum' going around 'l' is 'GLUM'.
(Other definitions for glum that I've seen before include "Miserable" , "Sad and downcast" , "Doleful" , "Down at the mouth, gloomy" , "Downcast and gloomy (4)" .)