Powerful alkali (6)
I believe the answer is:
potash
'powerful alkali' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for potash that I've seen before include "feed in bed?" , "The strange pathos of a potassium compound" , "Agricultural chemical" , "Powerful alkali from burnt wood" , "Potassium compound used as fertiliser" .)