Agatha first? Not her. Some one else (7)
I believe the answer is:
another
'else' is the definition.
(thesaurus)
'agatha first? not her some one' is the wordplay.
'agatha first?' becomes 'a' (est letter of 'agatha').
'some' suggests taking the first letters.
'one' becomes 'er' (I've seen this in other clues).
The first letter of 'her' is 'h'.
'a'+'not'+'h'+'er'='ANOTHER'
(Other definitions for another that I've seen before include "second" , "On earth (anag.)" , "A further (item)" , "A different (thing)" , "encore?" .)