Oonagh, about 51, walloped lout (8)
I believe the answer is:
hooligan
'lout' is the definition.
(I've seen this before)
'oonagh about 51 walloped' is the wordplay.
I cannot quite understand how this works, but
'about' could be 'a' (abbreviation in Chambers) and 'a' is found in the answer.
'51' could be 'li' (Roman numerals) and 'li' is found within the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for hooligan that I've seen before include "A young ruffian" , "Antisocial rough" , "Destructive or badly-behaved youth" , "Lawless type" , "Young rough" .)