Moment back (6)
I believe the answer is:
second
I believe this is a double definition.
'moment' is the first definition.
(I know that moment can be written as second)
'back' is the second definition.
(both can mean to support an idea or proposal)
(Other definitions for second that I've seen before include "Brief moment" , "Moment; musical interval" , "Between first and third" , "Unit of time - after No 1" , "A boxer's corner assistant - not the first" .)