Hearts put in boiled gently and were glistening (9)

I believe the answer is:

shimmered

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'were glistening' is the definition.
I can't tell whether this defines the answer.

'hearts put in boiled gently' is the wordplay.
'hearts' becomes 'h' (abbreviation used in card games e.g. bridge).
'put in' is an insertion indicator (back).
'boiled gently' becomes 'simmered' (I've seen this in another clue).
'h' inserted inside 'simmered' is 'SHIMMERED'.

'and' acts as a link.

Can you help me to learn more?

(Another definition for shimmered that I've seen is " shone".)

I've seen this clue in The Independent.

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