Constant runner-up is more compact (6)
I believe the answer is:
closer
'more compact' is the definition.
'constant runner-up' is the wordplay.
'constant' becomes 'c' (mathematical abbreviation).
'runner-up' becomes 'loser'.
'c'+'loser'='CLOSER'
'is' is the link.
(Other definitions for closer that I've seen before include "not so remote" , "Less distant" , "Nearer or more penny-pinching" , "Not so far off" , "denser" .)