Concluded actors can be dull (8)
I believe the answer is:
overcast
'be dull' is the definition.
(I've seen this before)
'concluded actors' is the wordplay.
'concluded' becomes 'over' ('over' can be a synonym of 'concluded').
'actors' becomes 'cast' (I've seen this before).
'over'+'cast'='OVERCAST'
'can' acts as a link.
(Other definitions for overcast that I've seen before include "(Of weather) cloudy, dull" , "Gloomy like cloudy sky" , "Overacts, being gloomy" , "less than fine" , "Cloud-covered" .)