Prince on an hour after midnight? Legend! He ruled (5)
I believe the answer is:
priam
'legend he ruled' is the definition.
The definition suggests a singular noun which matches the answer.
'prince on an hour after midnight?' is the wordplay.
'prince' becomes 'pr'.
'on' says to put letters next to each other (in a down clue, letters appear on others).
'an hour after midnight?' becomes 'iam' (1am).
'pr'+'iam'='PRIAM'
Can you help me to learn more?
(Other definitions for priam that I've seen before include "The last king of Troy, in Greek mythology" , "Trojan king" , "Father of Paris in Troy" , "Old king" , "Old man of Paris" .)