It's like teacher going round sorting the mail (7)
I believe the answer is:
similar
'it's like' is the definition.
(synonyms)
'teacher going round sorting the mail' is the wordplay.
'teacher' becomes 'sir' (a male teacher might be addressed as 'sir').
'going round' is an insertion indicator (some letters go around others).
'sorting' indicates an anagram.
'mail' anagrammed gives 'mila'.
'sir' going around 'mila' is 'SIMILAR'.
(Other definitions for similar that I've seen before include "Comparable" , "Resembling, like" , "Appearing the same" , "Having a close resemblance to" , "Alike, resembling" .)