Deep cut (4)
'deep cut' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for gash that I've seen before include "surplus to requirements" , "Jagged cut" , "Deep open cut" , "Open wound" , "Long deep slash" .)
(Other definitions for laceration that I've seen before include "wound" , "cutting" , "Tearing (of skin)" , "rent" , "Tear" .)
(Other definitions for incision that I've seen before include "Cut made surgically into the body" , "Cut as made with surgeon's knife" , "A surgical cut into the body" , "Surgical ten across" , "Cut, gash" .)