'Reversion' cast and support (9)
I believe the answer is:
throwback
'reversion' is the definition.
(I know that reversion can be written as throwback)
'cast and support' is the wordplay.
'cast' becomes 'throw' (synonyms).
'and' means one lot of letters go next to another.
'support' becomes 'back' (to back is to support or endorse).
'throw'+'back'='THROWBACK'
(Other definitions for throwback that I've seen before include "someone showing old-fashioned tendencies" , "Atavism" , "Instance of reversion to ancestral type" , "retort" , "Return" .)