The restriction I'm enlightened about? (5)
I believe the answer is:
limit
'the restriction' is the definition.
(I've seen this before)
'i'm enlightened about?' is the wordplay.
'enlightened' becomes 'lit' (I've seen this before).
'about?' means one lot of letters goes inside another.
'im' put into 'lit' is 'LIMIT'.
(Other definitions for limit that I've seen before include "Furthest extent" , "Keep in" , "Ceiling" , "Extreme point" , "Confine to specific boundary" .)