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'm not certain 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 give a lot more credence to this answer).
'in' indicates putting letters inside.
'show' becomes 'reveal' (synonyms).
'rs' put 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" .)