Support a proposition for a short time (6)
I believe the answer is:
second
This clue could be a double definition.
'support a proposition' is the first definition.
'second' can be an answer for 'support' (eg to second an idea is to support it). I am not sure about the 'a proposition' bit.
'a short time' is the second definition.
(second is a kind of short time)
'for' acts as a link.
Can you help me to learn more?
(Other definitions for second that I've seen before include "Support - runner-up" , "another?" , "taking silver back" , "bit" , "Back-up; short time" .)