Gloomy main actor — heartless, even (6)
I believe the answer is:
leaden
'gloomy' is the definition.
(thesaurus)
'main actor heartless even' is the wordplay.
'main actor' becomes 'lead' (lead is a kind of actor. I am not sure about the 'main' bit.).
'heartless' says to hollow out the word (remove centre letters).
'even' with its centre taken out is 'en'.
'lead'+'en'='LEADEN'
Can you help me to learn more?
(Other definitions for leaden that I've seen before include "Heavy - plodding" , "Oppressive" , "Heavy - overcast" , "Heavy or slow, like dark-gray metal" , "Depressing - heavy" .)