Chopped deer with rabbit's head - that's messed up (5)
I believe the answer is:
erred
'messed up' is the definition.
'chopped deer with rabbit's head' is the wordplay.
'chopped' is an anagram indicator.
'with' is an insertion indicator.
'head' indicates taking the first letters.
The first letter of 'rabbits' is 'r'.
'deer' with letters rearranged gives 'ered'.
'ered' enclosing 'r' is 'ERRED'.
'that's' is the link.
(Other definitions for erred that I've seen before include "Made mistake, was incorrect" , "Was mistaken" , "Gone astray!" , "was inaccurate" , "Made mistakes" .)