Transport returning home carrying advanced alien (7)
I believe the answer is:
martian
'alien' is the definition.
(thesaurus)
'transport returning home carrying advanced' is the wordplay.
'transport' becomes 'tram' (tram is a kind of transport**).
'returning' shows that the letters should be reversed in order.
'home' becomes 'in' ('he's home' can mean 'he's in').
'carrying' indicates putting letters inside.
'advanced' becomes 'a'.
'tram' back-to-front is 'mart'.
'in' placed around 'a' is 'ian'.
'mart'+'ian'='MARTIAN'
(Other definitions for martian that I've seen before include "Sci-fi alien" , "Relating to one planet" , "One living on red planet" , "One of H. G. Wells's invaders" , "Extraterrestrial" .)