Back runner-up (6)
I believe the answer is:
second
I believe this is a double definition.
'back' is the first definition.
('second' can be a synonym of 'back')
'runner-up' is the second definition.
(I've seen this before)
(Other definitions for second that I've seen before include "Position of runner-up" , "Attendant in a duel or boxing match" , "Subordinate; moment" , "Moment; musical interval" , "Back-up; short time" .)