Gloomy duo upset Romeo (4)
I believe the answer is:
dour
'gloomy' is the definition.
(I've seen this before)
'duo upset romeo' is the wordplay.
'upset' is an anagram indicator.
'romeo' becomes 'R' (phonetic alphabet: alpha, bravo, charlie etc.).
'duo' is an anagram of 'dou'.
'dou'+'r'='DOUR'
(Other definitions for dour that I've seen before include "Stern and severe looking" , "Stern, humourless" , "Grim, morose" , "Stern-looking, forbidding" , "Of appearance, harsh and forbidding" .)