Choose alien type of fence (6)
I believe the answer is:
picket
'type of fence' is the definition.
'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 "Protesting striker" , "Kind of fence or striking workers" , "Strike line; stake" , "Pale" , "Group outside workplace encouraging strike action" .)