Salt judge put on egg (7)
I believe the answer is:
nitrate
'salt' is the definition.
(nitrate is a kind of salt)
'judge put on egg' is the wordplay.
'judge' becomes 'rate' (rating is a kind of judging).
'put on' means one lot of letters go next to another (some letters go on or after others).
'egg' becomes 'nit' (a nit is an insect egg).
'rate' put after 'nit' is 'NITRATE'.
(Other definitions for nitrate that I've seen before include "following 28 [SILVER]" , "salt" , "Natural or synthetic fertiliser" , "Compound" , "Fertiliser chemical" .)