What sounds like zero marks (5)
I believe the answer is:
nicks
'marks' is the definition.
(nick can mean to make a mark)
'what sounds like zero' is the wordplay.
'what sounds like' shows a homophone (sound like).
'zero' becomes 'nix' (synonyms).
'nix' is a homophone of 'NICKS'.
(Other definitions for nicks that I've seen before include "Small incisions; pinches" , "Prisons (sl.)" , "Notches - arrests" , "Steals (sl.)" , "Catches" .)