Crossword Genius

Unusually, I sport a felspar casing to protect fracture (7,2,5)

Ross

I believe the answer is:

plaster of paris

I'm a little stuck... Click here to teach me more about this clue!

'casing to protect fracture' is the definition.
I can't judge whether this definition defines the answer.

'unusually i sport a felspar' is the wordplay.
'unusually' indicates an anagram.
'i'+'sport'+'a'+'felspar'='isportafelspar'
'isportafelspar' with letters rearranged gives 'PLASTER OF PARIS'.

Can you help me to learn more?

(Other definitions for plaster of paris that I've seen before include "is needed for the cast?" , "What might make cast" , "material for cast" .)

I've seen this clue in The Telegraph.
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