Pierce with a horn; blood (4)
I believe the answer is:
gore
This clue is a double definition.
'pierce with a horn' is the first definition.
(I've seen this in another clue)
'blood' is the second definition.
(gore means blood or violence)
(Other definitions for gore that I've seen before include "Spilt blood" , "Blood on a piece let into a garment?" , "Pierce or wound with a horn or tusk" , "Clotted blood" , "Blood; stab with tusk" .)