Garden fertilizer (6)
I believe the answer is:
manure
'garden fertilizer' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for manure that I've seen before include "Animal excreta as fertiliser" , "Muck" , "Animal dung used as fertilizer" , "Dung as fertiliser" , "Natural animal fertiliser" .)