Turn back actor’s script (5)
I believe the answer is:
ogham
'script' is the definition.
(medieval alphabet)
'turn back actor's' is the wordplay.
'turn' becomes 'go' (a person's move in a board game).
'back' says the letters should be written backwards.
'actor' becomes 'ham' (ham is a kind of actor).
'go' in reverse letter order is 'og'.
'og'+'ham'='OGHAM'
(Other definitions for ogham that I've seen before include "Old Irish writing of carved lines and notches" , "Old Irish writing carved into stones" , "Old British characters" , "set of characters" , "Ancient Irish writing carved in stones" .)