It's said to make perfect (8)
I believe the answer is:
practice
'it's said to make perfect' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for practice that I've seen before include "Customary procedure" , "Exercise; professional business" , "'....... what you preach'" , "The exercise of a profession" , "It makes perfect, 'tis said" .)