Is it useful 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 they are linked in a way I don't understand?

I don't understand the rest of the clue.

Can you help me to learn more?

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

I've seen this clue in the Evening Standard.

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