Support alternative (6)
I believe the answer is:
second
This clue is a double definition.
'support' is the first definition.
(eg to second an idea is to support it)
'alternative' is the second definition.
(thesaurus)
(Other definitions for second that I've seen before include "Short time - back" , "Basic unit of time" , "Support, assist" , "jiffy" , "Back-up; short time" .)