Reverse support (4,2)
I believe the answer is:
back up
This clue is a double definition.
'reverse' is the first definition.
(thesaurus)
'support' is the second definition.
(synonyms)
(Other definitions for back up that I've seen before include "Copy of data; traffic jam (US); support" , "provide support" , "Copy of a file on a separate storage device" , "Support - reverse" , "Support from the rear?" .)