It's like sorting mail, sir (7)
I believe the answer is:
similar
'it's like' is the definition.
(synonyms)
'sorting mail sir' is the wordplay.
'sorting' indicates an anagram.
'mail'+'sir'='mailsir'
'mailsir' with letters rearranged gives 'SIMILAR'.
(Other definitions for similar that I've seen before include "Comparable" , "Alike, resembling" , "Of the same kind" , "Appearing the same" , "Akin" .)