Can upset the price of fertiliser (7)
I believe the answer is:
nitrate
'fertiliser' is the definition.
(I've seen this before)
'can upset the price' is the wordplay.
'can' becomes 'tin' (both are metal containers).
'upset' says the letters should be written backwards.
'the price' becomes 'rate' (I've seen this before).
'tin' written backwards gives 'nit'.
'nit'+'rate'='NITRATE'
'of' is the link.
(Other definitions for nitrate that I've seen before include "Tear tin up for fertiliser" , "Fertilizer" , "Compound" , "following 28 [SILVER]" , "Nattier (anag.)" .)