Nails, as a test (4)
I believe the answer is:
aces
'nails as a test' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for aces that I've seen before include "holes in one" , "They're high and low on cards" , "winning points" , "Top (or bottom) cards" , "Winning serves" .)