Area 51 ultimately wore down extraterrestrial (5)
I believe the answer is:
alien
'extraterrestrial' is the definition.
(synonyms)
'area 51 ultimately wore down' is the wordplay.
'area' becomes 'a' (maths abbreviation).
'51' becomes 'LI' (roman numeral).
'ultimately' suggests the final letters.
The last letters of 'wore down' are 'en'.
'a'+'li'+'en'='ALIEN'
(Other definitions for alien that I've seen before include "Someone from outer space?" , "Outlandish" , "Clanger?" , "movie!" , "Foreigner, outsider" .)