Back in a moment (6)
I believe the answer is:
second
I believe this clue is a double definition.
'back' is the first definition.
('second' can be a synonym of 'back')
'a moment?' is the second definition.
('second' can be a synonym of 'moment')
'in' acts as a link.
(Other definitions for second that I've seen before include "Assistant in the ring" , "Support - runner-up" , "Time for a supporter" , "Short period of time" , "A duellist's or boxer's assistant" .)