Solids hardened, so doc set to work (13)

I believe the answer is:

dodecahedrons

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'solids' is the definition.
(dodecahedron is a kind of solid)

'hardened so doc set to work' is the wordplay.
'hardened so doc' anagrams to 'DODECAHEDRONS'.
However, I'm unsure how the anagram is indicated.

Can you help me to learn more?

I've seen this clue in The Guardian.

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