Pierce with a horn (4)
I believe the answer is:
gore
'pierce with a horn' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for gore that I've seen before include "Poetic blood from Al?" , "Pierce with horn - blood" , "Showing guts" , "it's a queasy sight" , "Make a hole in" .)