Cross piece knocked over, revealing secret entrance (8)
I believe the answer is:
trapdoor
'secret entrance' is the definition.
'trapdoor' can be an answer for 'entrance' (I've seen this before). I am not certain of the 'secret' bit.
'cross piece knocked over' is the wordplay.
'cross' becomes 'rood' (term for a crucifix).
'piece' becomes 'part' (synonyms).
'knocked over' says the letters should be written backwards.
'rood'+'part'='roodpart'
'roodpart' back-to-front is 'TRAPDOOR'.
'revealing' is the link.
Can you help me to learn more?
(Other definitions for trapdoor that I've seen before include "stage opening?" , "Means of access set in floor or ceiling" , "Hinged floor opening" , "Its movement can cause sudden drop" , "from which Redgrave maybe escaped?" .)