That part of the month for mail sorting (7)
I believe the answer is:
decimal
'that part of' is the definition.
I can't tell whether this defines the answer.
'month for mail sorting' is the wordplay.
'month' becomes 'dec' (abbreviation for December).
'for' says to put letters next to each other.
'sorting' indicates an anagram.
'mail' is an anagram of 'imal'.
'dec'+'imal'='DECIMAL'
'the' acts as a link.
Can you help me to learn more?
(Other definitions for decimal that I've seen before include "Using powers of ten" , "of Number Ten" , "In base ten" , "Relating to ten" , "Claimed (anag) - a point" .)