How to start writing address - in blood! (4,1)

I believe the answer is:

type a

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'blood' is the definition.
(I know that type A is a type of blood group)

I can't explain the rest of the clue.

(Other definitions for type a that I've seen before include "what online solvers would do at 15" , "full of ambition and energy" , "Strong personality" , "ambitious sort" , "stress-inducing nature" .)

I've seen this clue in The Irish Times.

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