Dead-head plant after early signs of spring (5)

I believe the answer is:

sprue

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'spring' is the definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.

I don't understand the rest of the clue.

Can you help me to learn more?

(Other definitions for sprue that I've seen before include "Species (herb)" , "passage into mould" .)

I've seen this clue in The Times and The Sunday Times.

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