Be gloomy (4)
I believe the answer is:
mope
'be gloomy' is the definition.
(synonyms)
This is all the clue.
(Other definitions for mope that I've seen before include "Move listlessly and aimlessly" , "Misery guts" , "Feel dejected and listless" , "Brood over a poem" , "Be listless or depressed" .)