Colourless mineral, see, in vessel completely knocked over (6)
I believe the answer is:
gypsum
'colourless mineral' is the definition.
(gypsum is often colourless)
'see in vessel completely knocked over' is the wordplay.
'see' becomes 'spy' (spy can mean to see or observe).
'in' indicates putting letters inside.
'vessel' becomes 'mug' (mug is a kind of vessel).
'completely knocked over' says the letters should be written backwards (all letters reversed).
'spy' placed inside 'mug' is 'muspyg'.
'muspyg' written backwards gives 'GYPSUM'.
(Other definitions for gypsum that I've seen before include "Mineral used in making plaster of Paris" , "Ingredient of fertilizer" , "Mineral giving plaster of Paris" , "Alabaster" , "that provides material for casts" .)