Long cut (5)
(Other definitions for slash that I've seen before include "rent" , "Cut with sweeping strokes" , "Cut as with a sword - /" , "Greatly reduce (prices)" , "Make sharp, long cut" .)
'long cut' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for gash that I've seen before include "Gaping wound" , "Deep open cut" , "Open wound" , "Long deep slash" , "Cut deeply" .)