Back a very short time (6)
I believe the answer is:
second
I believe this is a double definition.
'back' is the first definition.
(both can mean to support an idea or proposal)
'a very short time' is the second definition.
(second is a unit of time)
(Other definitions for second that I've seen before include "Mo" , "Support, assist" , "A boxer's corner assistant - not the first" , "awarded the silver medal?" , "Runner-up; short time" .)