Burn in Hades? I'm merciless (6)
I believe the answer is:
simmer
'burn' is the definition.
(thesaurus)
'in hades? i'm merciless' is the wordplay.
'in' says the answer is hidden in the clue.
'SIMMER' is hidden in the letters of 'hades im merciless'.
(Other definitions for simmer that I've seen before include "Cook in gently boiling water" , "Boil slowly at low temperature" , "Bubble on a low gas" , "Keep just short of boiling point, water or temper" , "Cook just below boiling point" .)