Lift up posterior (4)
I believe the answer is:
rear
This clue is a triple definition!
'lift' is the first definition.
(I know that lift can be written as rear)
'up' is the second definition.
(thesaurus)
'posterior' is the third definition.
(I know that posterior can be written as rear)
(Other definitions for rear that I've seen before include "Buttocks - rise up" , "Rise up at the back" , "Of horse, rise up on hind legs" , "The back part of something" , "Bring up (children)" .)