Romeo's terribly gloomy (6)
I believe the answer is:
morose
'gloomy' is the definition.
(I've seen this before)
'romeo's terribly' is the wordplay.
'terribly' indicates an anagram.
'romeos' with letters rearranged gives 'MOROSE'.
(Other definitions for morose that I've seen before include "Sad" , "in gloomy mood" , "Glum or sour-tempered" , "Down in the dumps" , "in a grumpy mood" .)