In spring, laid up fertiliser (6)
I believe the answer is:
potash
'fertiliser' is the definition.
(I know that potash is a type of fertiliser)
'in spring laid up' is the wordplay.
'in' means one lot of letters goes inside another.
'spring' becomes 'hop' (hopping is a kind of springing).
'laid' becomes 'sat' (sitting is a kind of laying).
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'hop' placed around 'sat' is 'hsatop'.
'hsatop' written backwards gives 'POTASH'.
(Other definitions for potash that I've seen before include "Potassium compound used in agriculture and industry" , "Potassium compound used as fertiliser" , "Powerful alkali used in gardening" , "Compound used in agriculture and industry - shop at (anag)" , "Powerful alkali from burnt wood" .)