A score and a half! (6)
I believe the answer is:
thirty
'a score and a half' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for thirty that I've seen before include "Six times five" , "Sum of first four perfect squares" , "Number of across clues in this puzzle" , "Number of pieces of silver paid to Judas" , "The number of silver coins for betrayal" .)