Gives mail out, based on a score (9)
I believe the answer is:
vigesimal
'based on a score' is the definition.
(vigesimal means based on the number 20)
'gives mail out' is the wordplay.
'out' indicates anagramming the letters (out can mean wrong or inaccurate).
'gives'+'mail'='givesmail'
'givesmail' with letters rearranged gives 'VIGESIMAL'.
(Other definitions for vigesimal that I've seen before include "of twenty" , "all on base score" .)