Choose alien fence (6)
I believe the answer is:
picket
'fence' is the definition.
(picket fence is a kind of fence)
'choose alien' is the wordplay.
'choose' becomes 'pick' (picking is a kind of choosing).
'alien' becomes 'et' (extra terrestrial).
'pick'+'et'='PICKET'
(Other definitions for picket that I've seen before include "Group outside workplace encouraging strike action" , "Striker's line outside workplace" , "Protesting striker" , "Line of workers on strike duty" , "Wooden stake for a fence" .)