Exactly zero t-shirts etc. returned (4-2)
I believe the answer is:
spot-on
'exactly' is the definition.
(I've seen this before)
I don't understand the remainder of the clue.
(Other definitions for spot-on that I've seen before include "Informally precise, receiving no post" , "Perfect" , "Precisely, in informal manner" , "Precisely what's required" , "Informally - exactly so, precisely" .)