Jackson's bill (6)
I believe the answer is:
twenty
'jackson's bill' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for twenty that I've seen before include "made by Q on a light blue square" , "Score, XX" , "previous clue?" , "John B Keane wrote about many young men of this age" , "This twice denotes normal vision" .)