Hence a light fizzled within (8)
I believe the answer is:
thalamus
I'm afraid I can't even guess how this answer relates to the clue.
Can you help me to learn more?
(Other definitions for thalamus that I've seen before include "Sensory part of brain; a lush mat (anag.)" , "bit of brain" , "Grey matter at base of brain" , "a bit of brains" .)