Fertiliser can's rising cost (7)
I believe the answer is:
nitrate
'fertiliser' is the definition.
(I've seen this before)
'can's rising cost' is the wordplay.
'can' becomes 'tin' ('tin' can be a synonym of 'can').
'rising' says the letters should be written in reverse (going up in a down clue).
'cost' becomes 'rate' (rate is a kind of cost**).
'tin' backwards is 'nit'.
'nit'+'rate'='NITRATE'
(Other definitions for nitrate that I've seen before include "following 28 [SILVER]" , "Tear tin up for fertiliser" , "Nattier (anag.)" , "Fertilizer" , "Fertiliser chemical" .)