"Midnight! the outpost of advancing day! / The frontier town and ____ of night!” (Longfellow, Two Rivers) (7)

I believe the answer is:

citadel

(Other definitions for citadel that I've seen before include "where Salvationists meet" , "Urban fortress" , "secure place" , "Keep - castle" , "Urban stronghold - dialect (anag)" .)

I've seen this clue in The Times.

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