Sad face reflected jealous heart inside (4)
I believe the answer is:
glum
'sad' is the definition.
(I've seen this in another clue)
'face reflected jealous heart inside' is the wordplay.
'face' becomes 'mug' (informal term for face).
'reflected' shows that the letters should be reversed in order.
'heart' indicates the central letters.
'inside' is an insertion indicator.
The centre of 'jealous' is 'l'.
'mug' backwards is 'gum'.
'gum' going around 'l' is 'GLUM'.
(Other definitions for glum that I've seen before include "Downcast and gloomy (4)" , "In low spirits" , "Down at the mouth, gloomy" , "Miserable" , "Moody and depressed" .)