A blow that shook Nero (4)
I believe the answer is:
oner
'a blow' is the definition.
(I have seen 'Heavy blow ' mean 'oner' so perhaps 'blow' could also mean 'oner')
'shook nero' is the wordplay.
'shook' indicates an anagram.
'nero' anagrammed gives 'ONER'.
'that' is the link.
(Other definitions for oner that I've seen before include "Big blow" , "A quid" , "Hit" , "A pound coin" , "Remarkable thing" .)