Perfect score, or half a score (3)
I believe the answer is:
ten
This clue is a double definition.
'perfect score' is the first definition.
(I've seen this in another clue)
'half a score' is the second definition.
(I've seen this in another clue)
(Other definitions for ten that I've seen before include "Half a score" , "Five times two" , "PM's address, Downing St" , "Very young" , "Roman numeral X" .)