On the subject of actors, is dull (8)
I believe the answer is:
overcast
'dull' is the definition.
(as in dull weather)
'on the subject of actors' is the wordplay.
'on' becomes 'over' (over can mean on top of).
'the subject of actors' becomes 'cast' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'over'+'cast'='OVERCAST'
'is' acts as a link.
Can you help me to learn more?
(Other definitions for overcast that I've seen before include "State of the sky when it is covered by clouds" , "(Of skies) grey" , "Gloomy like cloudy sky" , "less than fine" , "(Of weather) cloudy, dull" .)