Sterilise nebulising apparatus last doctor left out (4)

I believe the answer is:

spay

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'sterilise' is the definition.
(I know that spay is a more specific form of sterilise)

I don't understand the remainder of the clue.

Can you help me to learn more?

(Other definitions for spay that I've seen before include "doctor" , "Sterilise (a female animal)" , "Neuter" , "Remove the ovaries of a female animal" , "Sterilize" .)

I've seen this clue in The Irish Times.

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