One letter, in sorting mail, is not much! (7)
I believe the answer is:
minimal
'not much' is the definition.
Both the answer and definition are adjectives. Maybe they are linked in a way I don't understand?
'one letter in sorting mail' is the wordplay.
'one letter' becomes 'm' ().
'sorting' is an anagram indicator.
'mail' is an anagram of 'imal'.
'm'+'in'+'imal'='MINIMAL'
'is' acts as a link.
Can you help me to learn more?
(Other definitions for minimal that I've seen before include "barely worth considering" , "Of the least possible size" , "Of a smallest amount" , "Very small, the least possible (7)" , "Least possible; negligible" .)