Half of a score ... or a perfect score (3)
I believe the answer is:
ten
This clue is a double definition.
'half of 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 "X-WORD" , "Roman numeral X" , "time" , "Base of decimal system" , "How many" .)