Unable to deliver sonnet, gutted - long to pen one line, on reflection (10)

I believe the answer is:

sterilised

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'line on reflection' is the definition.
The definition suggests a verb in its base form which matches the answer.

I cannot understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for sterilised that I've seen before include "immaculately clean" , "Like theatre equipment" , "Thoroughly cleaned" .)

I've seen this clue in The Guardian.

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