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21a. These are solid, hard rocks round back of cove behind swimmer (10)
- E - R - H - D - A

It looks like Tetrahedra to me - not sure how it works with the clue, though!

Thank you Joan - I see it now
Tetra (fish)
anagram of hard and E (back of cove) = Hedra
Tetrahedra = Solid shapes
Enjoy the rest of your day -
I have a streaming cold at the moment which makes me feel quite dozy and a bit slower than usual!