Calcium sulphate in French dressing (7,2,5)
I believe the answer is:
plaster of paris
'calcium sulphate' is the definition.
(I know that plaster of Paris is a type of calcium sulphate)
'french dressing' is the wordplay.
I cannot quite see how this works, but
'french' could be 'f' (abbreviation) and 'f' is found in the answer.
'dressing' could be 'plaster' (plaster is a type of medical dressing) and 'plaster' is found in the answer.
This may be the basis of the clue (or it may be nonsense).
'in' acts as a link.
Can you help me to learn more?
(Other definitions for plaster of paris that I've seen before include "What might make cast" , "material for cast" , "is needed for the cast?" .)