Rally postmen, everyone having lost heart (8)

I believe the answer is:

jamboree

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'rally' is the definition.
(jamboree can mean a gathering or rally)

I don't understand the rest of the clue.

(Other definitions for jamboree that I've seen before include "Lively gathering" , "Scout or Guide rally" , "beano" , "'Gala get-together, of scouts say (8)'" , "Spree" .)

I've seen this clue in The Guardian.

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