Glow that comes from unstable fuel cores (9)
I believe the answer is:
fluoresce
'glow that' is the definition.
'fluoresce' can be an answer for 'glow' (I've seen this before). I'm not sure about the remainder of the definition.
'unstable fuel cores' is the wordplay.
'unstable' is an anagram indicator.
'fuel'+'cores'='fuelcores'
'fuelcores' with letters rearranged gives 'FLUORESCE'.
'comes from' acts as a link.
Can you help me to learn more?
(Other definitions for fluoresce that I've seen before include "glow brightly" , "Emit light when irradiated" .)