19 down
River
creature’s
back
consisting
of
thin
plates
(7)
River creature’s back consisting of thin plates (7)
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The definition is at the end of the clue.
How the clue works
The definition is consisting of thin plates, giving LAMINAR. The wordplay takes R for "River" and ANIMAL for "creature", then reverses the whole lot — indicated by "back" — giving LAMINA + R reversed to RANIMAL, wait — more precisely, R + ANIMAL = RANIMAL, and reversed that's LAMINAR. Neat!