Half a score, or a perfect score (3)
I believe the answer is:
ten
I believe this clue is a double definition.
'half a score' is the first definition.
(I've seen this in another clue)
'a perfect score' is the second definition.
(I've seen this in another clue)
(Other definitions for ten that I've seen before include "Very young" , "Dudley Moore/Bo Derek film" , "honour" , "time" , "How many" .)