Yet another form that's even worse (5,5)
I believe the answer is:
third class
'even worse' is the definition.
Although both the answer and definition are singular nouns, I don't understand how they can define each other.
'yet another form' is the wordplay.
'yet another' becomes 'third' (I can't justify this - if you can you should give a lot more credence to this answer).
'form' becomes 'class' (synonyms).
'third'+'class'='THIRD-CLASS'
'that's' acts as a link.
Can you help me to learn more?
(Other definitions for third class that I've seen before include "Low grade" , "modest level" , "(not important)" , "kind of degree" , "Grade C" .)