I'm within illuminated boundary (5)
I believe the answer is:
limit
'boundary' is the definition.
('limit' can be a synonym of 'boundary')
'i'm within illuminated' is the wordplay.
'within' indicates putting letters inside.
'illuminated' becomes 'lit' ('lit' can be a synonym of 'illuminated').
'im' placed into 'lit' is 'LIMIT'.
(Other definitions for limit that I've seen before include "Curb" , "Confine within bounds" , "Put restriction on" , "Constrain" , "Curtail" .)