Keying error deleted note from blood description (4)
I believe the answer is:
typo
'keying error' is the definition.
'note from blood description' is the wordplay.
'note' becomes 'e' (musical note).
'from' is a deletion indicator.
'blood description' becomes 'typeo' (Type O is a blood group).
'typeo' with 'e' taken away is 'TYPO'.
'deleted' is the link.
This may not be correct. It may belong to another bit of the clue.
(Other definitions for typo that I've seen before include "Printer's mistake" , "Small printing error" , "Spelling mistake" , "Literal error in printed matter, in short" , "Mistake in printed matter, in short" .)