Fool to assess salt (7)
I believe the answer is:
nitrate
'salt' is the definition.
(nitrate is a kind of salt)
'fool to assess' is the wordplay.
'fool' becomes 'nit' (both can mean a stupid person).
'to assess' becomes 'rate' (rating is a kind of assessing).
'nit'+'rate'='NITRATE'
(Other definitions for nitrate that I've seen before include "Fertilizer" , "following 28 [SILVER]" , "Compound" , "salt" , "Tear tin up for fertiliser" .)