Nasty giant, somewhat gloomy (7)
I believe the answer is:
stygian
'gloomy' is the definition.
(I've seen this before)
'nasty giant somewhat' is the wordplay.
'somewhat' says the answer is hidden in the clue.
'STYGIAN' can be found hidden inside 'nasty giant'.
(Other definitions for stygian that I've seen before include "Dark and gloomy, like the mythological river Styx" , "underworld runner" , "Very dark (poetic)" , "Gloomy or murky after the river Styx" .)