Instant support (6)
I believe the answer is:
second
This clue is a double definition.
'instant' is the first definition.
(I know that instant can be written as second)
'support' is the second definition.
(eg to second an idea is to support it)
(Other definitions for second that I've seen before include "Behind" , "Short time; subordinate" , "Short space of time" , "Next to first" , "Mo" .)