Backing for last couple of actors in show (8)
I believe the answer is:
reversal
'backing for last' is the definition.
'reversal' can be an answer for 'backing' (thesaurus). I am unsure of the 'for last' bit.
'couple of actors in show' is the wordplay.
'couple of actors' becomes 'rs' (I can't justify this - if you can you should believe this answer much more).
'in' is an insertion indicator.
'show' becomes 'reveal' (revealing is a kind of showing).
'rs' placed into 'reveal' is 'REVERSAL'.
(Other definitions for reversal that I've seen before include "U-turn" , "that makes no difference to mum or dad" , "Turnabout" , "switch" , "About-turn" .)