A clean sheet? (3)
I believe the answer is:
nil
'a clean sheet?' is the definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.
This is the entire clue.
Can you help me to learn more?
(Other definitions for nil that I've seen before include "Non-existent" , "Nothing, no value" , "none" , "Latin zero" , "Nothing, zero" .)