Buck up at the back! (4)
I believe the answer is:
rear
This clue is a triple definition!
'buck' is the first definition.
(I've seen this before)
'up' is the second definition.
(thesaurus)
'at the back' is the third definition.
(I've seen this before)
(Other definitions for rear that I've seen before include "Foster" , "Raise - back" , "Jump" , "Bring up - back end" , "Stern" .)