I'm entering illuminated border (5)
I believe the answer is:
limit
'border' is the definition.
(thesaurus)
'i'm entering illuminated' is the wordplay.
'entering' indicates putting letters inside.
'illuminated' becomes 'lit' ('lit' can be a synonym of 'illuminated').
'im' put within 'lit' is 'LIMIT'.
(Other definitions for limit that I've seen before include "Keep in" , "Constraint" , "end" , "Outer extent" , "Furthest extent" .)