Back in an hour, early (4)
I believe the answer is:
rear
'back' is the definition.
(something's rear is its back)
'in an hour early' is the wordplay.
'in' says the answer is hidden in the clue.
'REAR' can be found hidden inside 'an hour early'.
(Other definitions for rear that I've seen before include "Bring up - the tail" , "Buck" , "Of horse, rise up on hind legs" , "early, long before now" , "Breed or bring up" .)