More than likely lad goes after rum (4-2)
I believe the answer is:
odds-on
'more than likely' is the definition.
(I've seen this before)
'lad goes after rum' is the subsidiary indication.
'lad' becomes 'son'.
'goes after' is a charade indicator (letters next to each other).
'rum' becomes 'odd' (I've seen this in other clues).
'son' after 'odd' is 'oddson'.
(Other definitions for odds-on that I've seen before include "Rated likely to win" , "With success more likely than failure - check with bookie" , "Very likely to happen" , "Bookie's assessment of success being more likely than failure" , "With success more likely as bookie would calculate" .)