Cross actor’s words a setback for this bit of the stage? (4-4)

'this bit of the stage?' is the definition.
The definition and answer can be both man-made objects as well as being singular nouns.
Perhaps you can see an association between them that I don't see?

'cross actor's words a setback' is the wordplay.
'cross' becomes 'rood' (term for a crucifix).
'actor's words' becomes 'part' (I can't explain this - if you can you should believe this answer much more).
'a setback' is a reversal indicator.
'rood'+'part'='roodpart'
'roodpart' back-to-front is 'TRAP-DOOR'.

'for' is the link.

Can you help me to learn more?