Enemy, possibly of the Earth? (3)
I believe the answer is:
foe
'enemy' is the definition.
(synonyms)
'possibly of the earth?' is the wordplay.
'possibly' indicates anagramming the letters.
'the earth?' becomes 'E'.
'of' is an anagram of 'fo'.
'fo'+'e'='FOE'
(Other definitions for foe that I've seen before include "Evil rival" , "Opposition" , "opposing side" , "friend never!" , "Opponent" .)