I am illuminated about extent (5)
I believe the answer is:
limit
'extent' is the definition.
(I know that limit is a type of extent)
'i am illuminated about' is the wordplay.
'i am' becomes 'I'm' (contraction).
'illuminated' becomes 'lit' (synonyms).
'about' means one lot of letters goes inside another.
'im' inserted into 'lit' is 'LIMIT'.
(Other definitions for limit that I've seen before include "border" , "As far as something can go" , "Furthest extent" , "Confine to specific boundary" , "verge" .)