A score of two nil? (6)
I believe the answer is:
twenty
'a score' is the definition.
(I've seen this before)
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for twenty that I've seen before include "previous clue?" , "Number" , "Dozen plus eight" , "This twice denotes normal vision" , "John B Keane wrote about many young men of this age" .)