Those in cast, otherwise selection of one (6)
I believe the answer is:
actors
'those' is the definition.
I can't tell whether this definition defines the answer.
'in cast otherwise selection of one' is the wordplay.
'in' means one lot of letters goes inside another.
'otherwise' indicates an anagram.
'selection of one' becomes 'or' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'cast' with letters rearranged gives 'acts'.
'acts' placed around 'or' is 'ACTORS'.
(Other definitions for actors that I've seen before include "The Gleeson boys, say" , "Cast" , "Players on stage" , "They 23 down" , "Theatrical performers" .)