The blow that was Nero’s undoing (4)
I believe the answer is:
oner
'the blow that' is the definition.
'oner' can be an answer for 'the blow' (I have seen 'Heavy blow ' mean 'oner' so perhaps 'blow' could also mean 'oner'). I'm unsure of the rest of the definition.
'nero's undoing' is the wordplay.
'undoing' indicates an anagram.
'nero' anagrammed gives 'ONER'.
'was' is the link.
Can you help me to learn more?
(Other definitions for oner that I've seen before include "A pound coin" , "Heavy blow; 1 note" , "Outstanding person" , "Unique person" , "Remarkable thing" .)