Back the runner-up (6)
I believe the answer is:
second
This clue is a double definition.
'back' is the first definition.
('second' can be a synonym of 'back')
'the runner-up' is the second definition.
(I've seen this before)
(Other definitions for second that I've seen before include "Support - runner-up" , "Subordinate; moment" , "bit" , "Moment; back" , "Not first team (6,6) - See 25" .)