Lack of quality spoiled 10 pens (9)
I believe the answer is:
cheapness
'lack of quality spoiled 10 pens' is the definition.
Although both the answer and definition are singular nouns, I can't see how one could define the other.
'lack of quality' is the wordplay.
'lack of' becomes 'cheap' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'quality' becomes 'ness' (I've seen this in other clues).
'cheap'+'ness'='CHEAPNESS'
Can you help me to learn more?
(Other definitions for cheapness that I've seen before include "Scan sheep (anag)" , "Poor quality" .)