Sealant for slushy love letter? (4)

I believe the answer is:

kiss

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'love letter?' is the definition.
The answer and definition can be both acts as well as being singular nouns.
Maybe there's a link between them I don't understand?

I don't understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for kiss that I've seen before include "Affectionate greeting" , "Act of osculation" , "x (that may be French)" , "Transient touch" , "Eg, smacker, peck" .)

I've seen this clue in The Irish Times.

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