Wayward offspring created this but it's not top quality (6-4)
I believe the answer is:
second-rate
'wayward offspring created this but' is the definition.
The definition suggests a singular noun but the answer is not.
'not top quality' is the wordplay.
'not top' becomes 'second' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'quality' becomes 'rate' (I've seen this before).
'second'+'rate'='SECOND-RATE'
'it's' acts as a link.
Can you help me to learn more?
(Other definitions for second-rate that I've seen before include "Substandard" , "Of mediocre quality" , "Inferior" , "Poor" , "Shoddy" .)