Heads of state are disproportionately unhappy (3)
I believe the answer is:
sad
'unhappy' is the definition.
('sad' is associated in meaning with 'unhappy')
'heads of state are disproportionately' is the wordplay.
'heads of' says to take the initial letters.
The initial letters of 'state are disproportionately' is 'SAD'.
(Other definitions for sad that I've seen before include "Sorrowful, unhappy" , "Disconsolate" , "low" , "grey" , "Blue" .)