Teacher gets mail - terribly alike (7)
I believe the answer is:
similar
'alike' is the definition.
(I know that alike can be written as similar)
'teacher gets mail terribly' is the wordplay.
'teacher' becomes 'sir' (a male teacher might be addressed as 'sir').
'gets' is an insertion indicator (gets can mean captures or absorbs).
'terribly' indicates an anagram.
'mail' is an anagram of 'mila'.
'sir' enclosing 'mila' is 'SIMILAR'.
(Other definitions for similar that I've seen before include "Akin" , "Having a close resemblance to" , "Of the same kind" , "Comparable" , "Appearing the same" .)