Back behind the right ear (4)
I believe the answer is:
rear
'back' is the definition.
(something's rear is its back)
'behind the right ear' is the wordplay.
'behind' says to put letters next to each other.
'the right' becomes 'r' (common abbreviation).
'r' next to 'ear' is 'REAR'.
(Other definitions for rear that I've seen before include "Buck" , "parent" , "Bring up (children)" , "Raise; behind" , "Hind part" .)