Alternative alien on earth (7)
I believe the answer is:
another
'alternative' is the definition.
(thesaurus)
'alien on earth' is the wordplay.
'alien' indicates anagramming the letters.
'on'+'earth'='onearth'
'onearth' with letters rearranged gives 'ANOTHER'.
(Other definitions for another that I've seen before include "On earth (anag.)" , "A further (item)" , "A different one" , "Player or person not yet named or selected" , "A different (thing)" .)