Unusually, I sport a felspar casing to protect fracture (7,2,5)
I believe the answer is:
plaster of paris
'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" .)