Better than even a rum issue (4-2)
I believe the answer is:
odds-on
'better than even' is the definition.
(I've seen this before)
'a rum issue' is the wordplay.
'a rum' becomes 'odd' (rum can mean strange or dubious).
'issue' becomes 'son' (issue can mean someone's offspring).
'odd'+'son'='ODDS-ON'
(Other definitions for odds-on that I've seen before include "Rated likely to win" , "With success more likely than failure - check with bookie" , "Bookie's assessment of success being more likely than failure" , "With success more likely than favour - ask bookie" , "Very likely to happen" .)