Arboreal sleepers (6)
I believe the answer is:
sloths
'arboreal sleepers' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for sloths that I've seen before include "Slow-moving animals of tropical S America" , "Slow-moving mammals" , "Very slow-moving creatures" , "They're slow-moving and hang upside down" .)