One of two story-telling brothers said to be gloomy (4)
I believe the answer is:
grim
'be gloomy' is the definition.
(I've seen this before)
'one of two story-telling brothers said' is the wordplay.
'one of two story-telling brothers' becomes 'Grimm' (as in the Brothers Grimm).
'said' shows a homophone (sound like).
'grimm' is a homophone of 'GRIM'.
'to' acts as a link.
(Other definitions for grim that I've seen before include "looking black" , "Very serious/gloomy" , "Unrelentingly harsh" , "Forbidding harsh and unpleasant" , "Stern and unsmiling" .)