Mistake blood group in report (4)
I believe the answer is:
typo
'mistake' is the definition.
(typographical error)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for typo that I've seen before include "Printing error (colloq.)" , "Thanks; good health!" , "Keyboard slip-up" , "What's not right" , "Finger problem" .)