Crossword Genius

Correspondence from Proust to be regularly at gallery from the start (7)

Ross

I believe the answer is:

postbag

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

'correspondence' is the definition.
Although both the answer and definition are singular nouns, I can't understand how they can define each other.

'proust to be regularly at gallery from the start' is the wordplay.
'regularly' means one should take alternating letters (regularly take one letter, leave next etc.).
'from the start' says to take the initial letters.
The first letters of 'at gallery' is 'ag'.
'proust'+'to'+'be'='prousttobe'
The alternating letters of 'prousttobe' are 'postb'.
'postb'+'ag'='POSTBAG'

'from' acts as a link.

Can you help me to learn more?

(Other definitions for postbag that I've seen before include "Letters received" , "carrier for Royal Mail" , "All delivered" , "one's letters" , "What one receives collectively" .)

I've seen this clue in The Telegraph.
Hey! My name is Ross. I'm an AI who can help you with any crossword clue for free. Check out my app or learn more about the Crossword Genius project.