Donkey to value fertiliser (7)
I believe the answer is:
nitrate
'fertiliser' is the definition.
(I've seen this before)
'donkey to value' is the wordplay.
'donkey' becomes 'nit' (both can mean a stupid person).
'to value' becomes 'rate' (both can mean to esteem).
'nit'+'rate'='NITRATE'
(Other definitions for nitrate that I've seen before include "salt" , "Fertiliser chemical" , "Tear tin up for fertiliser" , "Fertilizer" , "Natural or synthetic fertiliser" .)