Degenerate eating rubbish lacks mineral (4,4)
I believe the answer is:
rock salt
'mineral' is the definition.
(rock salt is a kind of mineral)
'degenerate eating rubbish lacks' is the wordplay.
'degenerate' becomes 'rot' (rotting is a kind of degenerating).
'eating' indicates putting letters inside.
'rubbish' indicates an anagram (a poor quality or rubbish spelling of the letters).
'lacks' with letters rearranged gives 'cksal'.
'rot' placed around 'cksal' is 'ROCK-SALT'.
(Other definitions for rock salt that I've seen before include "Halite" , "condiment" , "Mineral NaC One" , "flavouring" , "Mineral sodium chloride" .)