Landlord fights for holes in doors (6,5)

I believe the answer is:

letter boxes

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'holes in doors' is the definition.
The definition and answer can be both man-made objects as well as being plural nouns.
Maybe there's a link between them I don't understand?

'landlord fights' is the wordplay.
'landlord' becomes 'letter' (landlords let property).
'fights' becomes 'boxes'.
'letter'+'boxes'='LETTER-BOXES'

'for' acts as a link.

Can you help me to learn more?

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