Frank Cross knocks back order for porter?

I believe the answer is:

open door

I'm a little stuck... Click here to teach me more about this clue!

'order for porter?' is the definition.
The answer and definition can be both human attributes as well as being singular nouns.
Maybe they are linked in a way I don't understand?

'frank cross knocks back' is the wordplay.
'frank' becomes 'open' ('open' can be a synonym of 'frank').
'cross' becomes 'rood' (term for a crucifix).
'knocks back' shows that the letters should be reversed in order (I've seen 'knock back' mean this).
'rood' in reverse letter order is 'door'.
'open'+'door'='OPEN-DOOR'

Can you help me to learn more?

(Other definitions for open door that I've seen before include "Immigration policy" , "Unrestricted entrance" .)

I've seen this clue in the Irish Times.

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