Nurse's backside (4)
I believe the answer is:
rear
This clue is a double definition.
'nurse's' is the first definition.
(thesaurus)
'backside' is the second definition.
(I know that backside can be written as rear)
(Other definitions for rear that I've seen before include "Back; bring up (young)" , "Hind part" , "Rump" , "what can be brought up, finally" , "Bring up or stand up on hind legs" .)