Support runner-up (6)
I believe the answer is:
second
I believe this clue is a double definition.
'support' is the first definition.
(eg to second an idea is to support it)
'runner-up' is the second definition.
(I've seen this before)
(Other definitions for second that I've seen before include "additional > support" , "jiffy" , "Attendant in a duel or boxing match" , "Back-up; short time" , "Eighty six thousand and four hundredth of a day" .)