Get one's back up? (4)
I believe the answer is:
rear
'up?' is the definition.
(thesaurus)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for rear that I've seen before include "Hitchcock film set in one room" , "Bring up - the tail" , "Bring up - back end" , "Rare at the back" , "Stern" .)