Alien in a tram — ridiculous! (7)
I believe the answer is:
martian
'alien' is the definition.
(thesaurus)
'in a tram ridiculous' is the wordplay.
'ridiculous' is an anagram indicator.
'in'+'a'+'tram'='inatram'
'inatram' is an anagram of 'MARTIAN'.
(Other definitions for martian that I've seen before include "Being from another planet" , "Relating to a particular planet" , "seeing little green man?" , "Sci-fi alien" , "Sci-fi character" .)