Gloomy myself, or so troubled at heart (6)
I believe the answer is:
morose
'gloomy' is the definition.
(morose means gloomy or sad)
'myself or so troubled at heart' is the wordplay.
'myself' becomes 'me'.
'troubled' indicates anagramming the letters.
'at heart' is an insertion indicator (inserted letters go to the heart of the word).
'or'+'so'='orso'
'orso' anagrammed gives 'oros'.
'me' placed around 'oros' is 'MOROSE'.
(Other definitions for morose that I've seen before include "Lugubrious" , "Silent, gloomy and ill-tempered" , "Miserable" , "Sad" , "Rarely" .)