Support; rear surface (4)
I believe the answer is:
back
This clue is a double definition.
'support' is the first definition.
(to back is to support or endorse)
'rear surface' is the second definition.
(I've seen this before)
(Other definitions for back that I've seen before include "See 12" , "Support - football player" , "Fashionable again" , "Help!" , "part of the body" .)