One’s entirely a hoodlum? Or not? (5)
I believe the answer is:
yahoo
'one's' is the definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.
'entirely a hoodlum? or not?' is the wordplay.
'YAHOO' is hidden within the letters.
But, I'm not sure how this is indicated.
(Other definitions for yahoo that I've seen before include "Ugly swipe at delivery" , "expression of joy" , "One of Dean Swift's brutish people" , "Uncivilised brute(Swift)" , "Exclamation of joy; brute" .)