Not fair on the subject of actors (8)
I believe the answer is:
overcast
'not fair' is the definition.
(I've seen this before)
'the subject of actors' is the wordplay.
'the subject of' becomes 'over' (I can't explain this - if you can you should give a lot more credence to this answer).
'actors' becomes 'cast' (I've seen this before).
'over'+'cast'='OVERCAST'
'on' 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 sky) cloudy, dull" , "Sunless" , "State of a dark cloudy sky" , "Gloomy like cloudy sky" .)