Extra players being dull (8)
I believe the answer is:
overcast
'being dull' is the definition.
(I've seen this before)
'extra players' is the wordplay.
'extra' becomes 'over'.
'players' becomes 'cast' (I've seen this before**).
'over'+'cast'='OVERCAST'
(Other definitions for overcast that I've seen before include "not entirely fair" , "The sky covered by dark clouds" , "(Of sky) cloudy, dull" , "Sunless" , "Cloud-covered" .)