Language used in the first mail sorting (5)
I believe the answer is:
tamil
'language' is the definition.
(I know that Tamil is a type of Tamil languages)
'the first mail sorting' is the wordplay.
'the first' becomes 't' (1st letter of 'the').
'sorting' indicates anagramming the letters.
'mail' is an anagram of 'amil'.
't'+'amil'='TAMIL'
'used in' acts as a link.
(Other definitions for tamil that I've seen before include "One in race" , "Perhaps Indian" , "Language of southern India and Sri Lanka" , "Native of south India or Sri Lanka" , "South Indian people or language" .)