Where my letters are delivered in volume (4)

I believe the answer is:

tome

'volume' is the definition.
(I've seen this before)

I can't explain the remainder of the clue.

(Other definitions for tome that I've seen before include "Large scholarly volume" , "Big book" , "Heavy volume" , "War and Peace, say" , "Large, weighty book" .)

I've seen this clue in The Guardian.

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