Blood group often reported erroneously at first (4)
I believe the answer is:
gore
'blood' is the definition.
(gore means blood or violence)
'group often reported erroneously at first' is the wordplay.
'at first' suggests taking the first letters.
The initial letters of 'group often reported erroneously' is 'GORE'.
(Other definitions for gore that I've seen before include "Bloodshed" , "Pierce with horn - blood" , "it's a queasy sight" , "Blood on a piece let into a garment?" , "Blood; stab with tusk" .)