Spare back numbers? More likely than not (4-2)
I believe the answer is:
odds-on
'more likely than not' is the definition.
(I've seen this before)
'spare back numbers?' is the wordplay.
'spare' becomes 'odd' (I've seen this in other clues).
'back numbers?' becomes 'son' (I can't explain this - if you can you should give a lot more credence to this answer).
'odd'+'son'='ODDS-ON'
Can you help me to learn more?
(Other definitions for odds-on that I've seen before include "More likely to win than lose" , "Better than evens" , "With favourable chances" , "With success more likely than favour - ask bookie" , "High probability" .)