Mum more properly binds leg for one in the long run (10)

'one in the long run' is the definition.
(a marathon is a long run)

'mum more properly binds leg' is the wordplay.
'mum' becomes 'ma' (both can mean mother).
'more properly' becomes 'rather' (rather could mean more properly or justly).
'binds' means one lot of letters goes inside another.
'leg' becomes 'on' (cricket term, fielding positions).
'rather' placed around 'on' is 'rathoner'.
'ma'+'rathoner'='MARATHONER'

'for' acts as a link.