Common fertilizer (7)
I believe the answer is:
nitrate
'common fertilizer' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for nitrate that I've seen before include "salt" , "Natural or synthetic fertiliser" , "Nattier (anag.)" , "Fertiliser chemical" , "following 28 [SILVER]" .)