Sleep in the cosiest apartment (6)
I believe the answer is:
siesta
'sleep' is the definition.
(I've seen this before)
'in the cosiest apartment' is the wordplay.
'in' says the answer is hidden in the clue.
'SIESTA' can be found hidden inside 'cosiest apartment'.
(Other definitions for siesta that I've seen before include "Afternoon nap in Cadiz" , "A Spanish mid-day nap" , "Catnap" , "Afternoon nap in a warm climate" , "PM snoozing?" .)