Older egg is bad? Nonsense! (8)
I believe the answer is:
doggerel
'nonsense' is the definition.
(I've seen this before)
'older egg is bad?' is the wordplay.
'is bad?' indicates anagramming the letters (I've seen 'being bad' mean this).
'older'+'egg'='olderegg'
'olderegg' anagrammed gives 'DOGGEREL'.
(Other definitions for doggerel that I've seen before include "Very bad poetry or verse" , "Nonsense verse" , "Rubbishy verse" , "Unpretentious verse" , "not great poetry" .)