Alien without alternative means to act (5)
I believe the answer is:
feign
'to act' is the definition.
(feigning is a kind of acting)
'alien without alternative' is the wordplay.
'alien' becomes 'foreign' (synonyms).
'without' is a deletion indicator.
'alternative' becomes 'or'.
'foreign' with 'or' taken away is 'FEIGN'.
'means' acts as a link.
(Other definitions for feign that I've seen before include "Counterfeit, simulate" , "outdated shape" , "Simulate, fake" , "Pretend to experience" , "Copy" .)