The invading yellow alien (5)
I believe the answer is:
other
'alien' is the definition.
(thesaurus)
'the invading yellow' is the wordplay.
'invading' is an insertion indicator.
'yellow' becomes 'or' (heraldic term).
'the' going inside 'or' is 'OTHER'.
(Other definitions for other that I've seen before include "Second" , "not like you" , "else!" , "Not the one mentioned" , "Remaining" .)