Some of the drowsiest animals rest (6)
I believe the answer is:
siesta
'rest' is the definition.
(thesaurus)
'some of the drowsiest animals' is the wordplay.
'some of' indicates the answer is hidden within the clue.
'SIESTA' is hidden within 'drowsiest animals'.
(Other definitions for siesta that I've seen before include "Afternoon rest" , "Afternoon relaxation" , "An afternoon nap in Spain" , "A Spanish mid-day nap" , "Rest in the heat of the day" .)