Fertiliser mixed in treat (7)
I believe the answer is:
nitrate
'fertiliser' is the definition.
(I've seen this before)
'mixed in treat' is the wordplay.
'mixed' indicates an anagram.
'in'+'treat'='intreat'
'intreat' is an anagram of 'NITRATE'.
(Other definitions for nitrate that I've seen before include "following 28 [SILVER]" , "Compound" , "Fertiliser chemical" , "Nattier (anag.)" , "Tear tin up for fertiliser" .)