Crossword Genius

Restful as a noted Lagoon (6)

Ross

I believe the answer is:

sleepy

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

I don't understand how the remainder of the clue works.

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(Other definitions for sleepy that I've seen before include "(Of place) low in activity" , "Drowsy; indolent" , "Doc's nodding pal" , "Drowsy - without excitement" , "Dwarf" .)

I've seen this clue in the Evening Standard.
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