Popular Star Wars character, unwelcome visitor (7)
I believe the answer is:
invader
'unwelcome visitor' is the definition.
(an invader is an unwelcome intruder)
'popular star wars character' is the wordplay.
'popular' becomes 'in' ('in' can mean trendy).
'star wars character' becomes 'Vader' (Darth Vader).
'in'+'vader'='INVADER'
(Other definitions for invader that I've seen before include "One entering by force" , "One entering as an enemy" , "Military intruder" , "Attacker from abroad" , "One illegally entering" .)