Trojan prince's dressing in gypsum (7,2,5)
I believe the answer is:
plaster of paris
'gypsum' is the definition.
(I've seen this in another clue)
'trojan prince's dressing' is the wordplay.
I cannot really see how this works, but
'trojan' could be 'paris' (mythical Trojan) and 'paris' is present in the answer.
'dressing' could be 'plaster' (plaster is a type of medical dressing) and 'plaster' is found in the answer.
The remaining letters 'of' is a valid word which might be clued in a way I don't see.
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 "material for cast" , "What might make cast" , "is needed for the cast?" .)