I'm illuminated without constraint (5)
I believe the answer is:
limit
'constraint' is the definition.
(I've seen this in another clue)
'i'm illuminated without' is the wordplay.
'illuminated' becomes 'lit' (synonyms).
'without' means one lot of letters goes inside another.
'im' going inside 'lit' is 'LIMIT'.
(Other definitions for limit that I've seen before include "verge" , "Boundary or maximum" , "Curtail" , "Boundary, demarcation line" , "Point beyond which one should not go" .)