Boss eats horribly toxic substance (8)
I believe the answer is:
asbestos
'toxic substance' is the definition.
'asbestos' can be an answer for 'substance' (asbestos is a kind of substance). I'm not certain of the 'toxic' bit.
'boss eats horribly' is the wordplay.
'horribly' is an anagram indicator (a horribly wrong version of the letters).
'boss'+'eats'='bosseats'
'bosseats' anagrammed gives 'ASBESTOS'.
Can you help me to learn more?
(Other definitions for asbestos that I've seen before include "dangerous dust" , "Old fire-retardant" , "Fire defence" , "Firewall material" , "Fireproof material, now known to be dangerous" .)