Ditch colouring material containing potassium (4)
I believe the answer is:
dyke
'ditch' is the definition.
(I've seen this before)
'colouring material containing potassium' is the wordplay.
'colouring material' becomes 'dye' (I've seen this before).
'containing' indicates putting letters inside.
'potassium' becomes 'k' (K is the chemical symbol for potassium).
'dye' going around 'k' is 'DYKE'.
(Other definitions for dyke that I've seen before include "Ditch; embankment" , "Offa's barrier?" , "Flood barrier" , "Barrier against water" , "Ditch; earthwork" .)