Alkaline compound (6)
I believe the answer is:
potash
'alkaline compound' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for potash that I've seen before include "Potassium compound used in agriculture and industry" , "Type of fertiliser" , "chemical compound" , "The strange pathos of a potassium compound" , "feed in bed?" .)