Ways to escape and duck headless alien (8)
I believe the answer is:
outsider
'alien' is the definition.
(I know that outsider is a type of alien)
'ways to escape and duck headless' is the wordplay.
'ways to escape' becomes 'outs' (I've seen this in another clue).
'and' means one lot of letters go next to another.
'duck' becomes 'eider duck' (eider is a type of duck).
'headless' means to remove the first letter.
'eider' with its initial letter removed is 'ider'.
'outs'+'ider'='OUTSIDER'
(Other definitions for outsider that I've seen before include "Unfancied competitor" , "One not in a particular group" , "Improbable winner" , "Foreigner" , "Stranger" .)