It tells us when a fresh batch of mail gets in (4,6)
I believe the answer is:
time signal
'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'.
Can you help me to learn more?
(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" .)