Love once forsworn by sullen sleuth (5)

I believe the answer is:

morse

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'love' is the definition.
Although both the answer and definition are singular nouns, I can't understand how they can define each other.

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

Can you help me to learn more?

(Other definitions for morse that I've seen before include "Fictional inspector; code" , "One who enjoyed solving code" , "Code; TV detective" , "Oxford detective" , "Dot-dash code inventor" .)

I've seen this clue in The Times.

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