Crossword Genius

Gentle heating retains hot glow (7)

Ross

I believe the answer is:

shimmer

I'm a little stuck... Click here to teach me more about this clue!

'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?)" .)

I've seen this clue in The Sun.
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