Hollow footwear for an alien (6)
I believe the answer is:
socket
'hollow' is the definition.
(thesaurus)
'footwear for an alien' is the wordplay.
'footwear' becomes 'sock' (sock is a kind of footwear).
'for' is a charade indicator (letters next to each other).
'an alien' becomes 'et' (extra terrestrial).
'sock'+'et'='SOCKET'
(Other definitions for socket that I've seen before include "Power point" , "Plug receiver" , "Light bulb holder" , "Small crown" , "Counterpart to a plug" .)