Competent at arithmetic (8)
I believe the answer is:
numerate
'competent at arithmetic' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for numerate that I've seen before include "Good at arithmetic" , "Having a good knowledge of maths" , "Well versed in maths" , "Able to do arithmetic" , "Arithmetically competent" .)