Crossword Genius

Material supplied for cast following break (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!

'material' is the definition.
(plaster of Paris is a kind of material)

'supplied for cast following break' is the wordplay.
I cannot really see how this works, but
an anagram of 'for' is 'rof' which is present in the answer.
This explanation may well be incorrect...

Can you help me to learn more?

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

I've seen this clue in The Telegraph.
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