Sleep extremely peacefully in a diaper (5)
I believe the answer is:
nappy
'a diaper' is the definition.
('nappy' can be a synonym of 'diaper')
'sleep extremely peacefully' is the wordplay.
'sleep' becomes 'nap' (a nap is a short sleep).
'extremely' means to remove the middle letters (only the extremes of the word are used).
'peacefully' with its middle taken out is 'py'.
'nap'+'py'='NAPPY'
'in' acts as a link.
(Other definitions for nappy that I've seen before include "Item for baby" , "Baby garment" , "Baby's garment" , "Diaper" , "Baby wear" .)