See in cup knocked over a soft mineral (6)
I believe the answer is:
gypsum
'a soft mineral' is the definition.
(I've seen this before)
'see in cup knocked over' is the wordplay.
'see' becomes 'spy' (I can't justify this - if you can you should give a lot more credence to this answer).
'in' means one lot of letters goes inside another.
'cup' becomes 'mug' (I've seen this before).
'knocked over' is a reversal indicator.
'spy' going inside 'mug' is 'muspyg'.
'muspyg' in reverse letter order is 'GYPSUM'.
Can you help me to learn more?
(Other definitions for gypsum that I've seen before include "Ingredient of fertilizer" , "Mineral used in making plaster of Paris" , "Chalky mineral from which plaster of Paris is made" , "Alabaster" , "Chalky material in plaster of Paris" .)