Like sorting mail, sir! (7)
I believe the answer is:
similar
'like' is the definition.
(synonyms)
'sorting mail sir' is the wordplay.
'sorting' is an anagram indicator.
'mail'+'sir'='mailsir'
'mailsir' with letters rearranged gives 'SIMILAR'.
(Other definitions for similar that I've seen before include "Akin" , "Having a close resemblance to" , "Appearing the same" , "Resembling, like" , "Comparable" .)