Nap in noisiest apartment (6)
I believe the answer is:
siesta
'nap' is the definition.
(siesta is a kind of nap)
'in noisiest apartment' is the wordplay.
'in' indicates the answer is hidden within the clue.
'SIESTA' is hidden in the letters of 'noisiest apartment'.
(Other definitions for siesta that I've seen before include "An afternoon nap in Spain" , "Catnap" , "PM snoozing?" , "Early afternoon nap" , "A Spanish mid-day nap" .)