Treat in new fertiliser (7)
I believe the answer is:
nitrate
'fertiliser' is the definition.
(I've seen this before)
'treat in new' is the wordplay.
'new' indicates an anagram.
'treat'+'in'='treatin'
'treatin' with letters rearranged gives 'NITRATE'.
(Other definitions for nitrate that I've seen before include "Fertilizer" , "Tear tin up for fertiliser" , "salt" , "Nattier (anag.)" , "Fertiliser chemical" .)