Nattier form of fertiliser (7)
I believe the answer is:
nitrate
'fertiliser' is the definition.
(I've seen this before)
'nattier form' is the wordplay.
'form' indicates an anagram (a different form of the same letters).
'nattier' is an anagram of 'NITRATE'.
'of' is the link.
(Other definitions for nitrate that I've seen before include "Fertilizer" , "following 28 [SILVER]" , "salt" , "Compound" , "Tear tin up for fertiliser" .)