Likely lad after rum (4-2)
I believe the answer is:
odds-on
'likely' is the definition.
(thesaurus)
'lad after rum' is the subsidiary indication.
'lad' becomes 'son'.
'after' says to put 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 favourable chances" , "With success more likely than failure - check with bookie" , "High probability" , "With success more likely than favour in bookie's phrase" .)