22 across
Hide
from
source
of
loud
beastly
sound
that
is
piercing
(3,3)
Hide from source of loud beastly sound that is piercing (3,3)
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The definition is at the beginning of the clue.
How the clue works
The definition is hide from, cluing a phrase meaning to stay out of sight and keep your head down. You've got L, the "source of loud", then beastly sound gives you LOW (a cow's moo), and tucked inside is IE — that is, as in i.e. — piercing it to give you L-IE-LOW.