When to do a Masters with German letters here (7)

I believe the answer is:

postbag

'letters here' is the definition.
(bag containing letters sent in the post)

I cannot understand how the remainder of the clue works.

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

I've seen this clue in The Independent.

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