Undesirable to expose actors (7)
I believe the answer is:
outcast
'undesirable' is the definition.
(thesaurus)
'expose actors' is the wordplay.
'expose' becomes 'out' (to out someone is to expose them).
'actors' becomes 'cast' (group of actors).
'out'+'cast'='OUTCAST'
'to' acts as a link.
(Other definitions for outcast that I've seen before include "Role of Ishmael" , "Derelict" , "Rejected individual" , "one's not admitted" , "untouchable" .)