What to change when baby's sleepy? (5)

I believe the answer is:

nappy

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'sleepy?' is the definition.
Both the answer and definition are adjectives. Maybe you can see an association between them that I don't see?

I cannot understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for nappy that I've seen before include "Baby garment" , "One pinned on baby" , "Baby's garment" , "Pad for baby" , "Baseball fielder" .)

I've seen this clue in the Evening Standard.

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