Potassium compound used as fertiliser (6)
I believe the answer is:
potash
'potassium compound used as fertiliser' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for potash that I've seen before include "Agricultural chemical" , "Potassium carbonate or hydroxide" , "Compound used in agriculture and industry - shop at (anag)" , "Powerful alkali from burnt wood" , "The strange pathos of a potassium compound" .)