21/22 across
Parents
maybe
initiate
corresponding
with
school’s
head
(5,9)
Parents maybe initiate corresponding with school’s head (5,9)
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The definition is at the beginning of the clue.
How the clue works
Look at Parents maybe — that's your definition, since parents are one example of blood relatives. The wordplay builds the answer piece by piece: BLOOD means "to initiate", then RELATIVE gives you "corresponding", and finally S is the head (first letter) of "school".