Behind, in arrears (4)
I believe the answer is:
rear
'behind' is the definition.
(both can mean one's bottom)
'in arrears' is the wordplay.
'in' says the answer is hidden in the clue.
'REAR' is hidden within 'arrears'.
(Other definitions for rear that I've seen before include "Bring up (children)" , "Stern" , "Jump" , "parent" , "Bring up - the tail" .)