Gentle heating retains hot glow (7)
I believe the answer is:
shimmer
'glow' is the definition.
(I've seen this before)
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for shimmer that I've seen before include "Twinkle" , "glow" , "glint"" , "Tinsel might do it" , "Glisten (on water?)" .)