Bring up what is behind (4)
I believe the answer is:
rear
I believe this is a triple definition!
'bring' is the first definition.
(thesaurus)
'up' is the second definition.
(thesaurus)
'what' is a link.
'is behind' is the third definition.
(both can mean one's bottom)
(Other definitions for rear that I've seen before include "Buttocks - rise up" , "Raise - back" , "parent" , "Rise up at the back" , "Bring up or stand up on hind legs" .)