Behind, due to a rare mix-up! (4)
I believe the answer is:
rear
'behind' is the definition.
(both can mean one's bottom)
'a rare mix-up' is the wordplay.
'mix-up' indicates anagramming the letters.
'rare' anagrammed gives 'REAR'.
'due to' acts as a link.
(Other definitions for rear that I've seen before include "Jump" , "Raise; behind" , "Posterior" , "Hindmost part" , "parent" .)