Not entirely gloomy, this old story? (4)
I believe the answer is:
myth
'old story?' is the definition.
(I've seen this before)
'not entirely gloomy this' is the wordplay.
'not entirely' indicates the answer is hidden within the clue.
'MYTH' is hidden in the letters of 'gloomy this'.
(Other definitions for myth that I've seen before include "There's no truth in it" , "Legend; fable" , "Fictitious thing; traditional story" , "Fanciful story" , "Traditional allegory" .)