Rarely treat in salt (7)
I believe the answer is:
nitrate
'salt' is the definition.
(nitrate is a kind of salt)
'rarely treat in' is the wordplay.
'rarely' indicates an anagram (rare can mean unusual or strange).
'treat'+'in'='treatin'
'treatin' anagrammed gives 'NITRATE'.
(Other definitions for nitrate that I've seen before include "Tear tin up for fertiliser" , "following 28 [SILVER]" , "Compound" , "Fertiliser chemical" , "salt" .)