Reject striking actors (7)
I believe the answer is:
'reject' is the definition.
(I've seen this before)
'striking actors' is the wordplay.
'striking' becomes 'out' (striking out is a common phrase).
'actors' becomes 'cast' (I've seen this in another clue).
(Other definitions for outcast that I've seen before include "Pariah, leper", "In one of the islands, Joseph wrote", "'Person excluded from society, a pariah (7)'", "vagabond", "One excluded from group".)