It tells us when a fresh batch of mail gets in (4,6)

I believe the answer is:

time signal

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'it tells us when a fresh batch' is the definition.
The definition suggests a singular noun which matches the answer.

'of mail gets in' is the wordplay.
'of' indicates anagramming the letters (the answer comes from or 'of' the letters).
'mail'+'gets'+'in'='mailgetsin'
'mailgetsin' is an anagram of 'TIME-SIGNAL'.

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(Other definitions for time signal that I've seen before include "beeps, perhaps" , "eg Six pips" , "Electronic indication of the correct hour" , "up-to-the minute sound" , "Eg, Greenwich pips" .)

I've seen this clue in The Globe and Mail.

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