Cross section pulled back to reveal hidden entrance (4,4)
I believe the answer is:
trap door
'hidden entrance' is the definition.
(door hidden in a floor)
'cross section pulled back' is the wordplay.
'cross' becomes 'rood' (term for a crucifix).
'section' becomes 'part' (synonyms).
'pulled back' says the letters should be written in reverse.
'rood'+'part'='roodpart'
'roodpart' back-to-front is 'TRAP-DOOR'.
'to reveal' is the link.
(Other definitions for trap door that I've seen before include "Means of entry through floor or ceiling" , "Hinged exit in floor or ceiling" , "Opening set in a stage or ceiling" , "Opening in roof or floor" , "Hinged flap in floor" .)