Hit an alien ordered to exterminate (10)
I believe the answer is:
annihilate
'exterminate' is the definition.
(thesaurus)
'hit an alien ordered' is the wordplay.
'ordered' indicates anagramming the letters.
'hit'+'an'+'alien'='hitanalien'
'hitanalien' anagrammed gives 'ANNIHILATE'.
'to' is the link.
(Other definitions for annihilate that I've seen before include "Utterly destroy" , "Completely defeat" , "Wipe out" .)