Booming English lad in verbal onslaught (8)
I believe the answer is:
resonant
'booming' is the definition.
(as in a booming sound)
'english lad in verbal onslaught' is the wordplay.
'english' becomes 'E' (abbreviation as in OED).
'lad' becomes 'son' (lad can informally mean a young boy).
'in' indicates putting letters inside.
'verbal onslaught' becomes 'rant' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'e'+'son'='eson'
'eson' put inside 'rant' is 'RESONANT'.
Can you help me to learn more?
(Other definitions for resonant that I've seen before include "Re-echoing" , "producing vibrant sound" , "Booming" , "Deeply sounding" , "Deep, clear and continuing to sound or reverberate" .)