Sue illuminated one entrance (8)
I believe the answer is:
litigate
'sue' is the definition.
(I know that sue can be written as litigate)
'illuminated one entrance' is the wordplay.
'illuminated' becomes 'lit' (synonyms).
'one' becomes 'i' (Roman numeral).
'entrance' becomes 'gate' (type of entrance).
'lit'+'i'+'gate'='LITIGATE'
(Other definitions for litigate that I've seen before include "Go to law" , "Engage in legal proceedings" , "Sue" , "take action" , "Dispute by law" .)