Restful as a noted Lagoon (6)
I believe the answer is:
sleepy
'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.
Can you help me to learn more?
(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" .)