Totally nails (4)
I believe the answer is:
aces
'totally nails' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for aces that I've seen before include "excellent service" , "Ones or elevens in cards" , "that are non-returnable" , "never come back" , "Experts (in their field)" .)