Likely to happen for strange boy (4-2)
I believe the answer is:
odds-on
'likely to happen' is the definition.
(I've seen this before)
'strange boy' is the subsidiary indication.
'strange' becomes 'odd' (I've seen this before).
'boy' becomes 'son' ('son' can be a synonym of 'boy'**).
'odd'+'son'='oddson'
'for' is the link.
(Other definitions for odds-on that I've seen before include "With favourable chances" , "Better than evens" , "A better than even chance of success" , "With success more likely than favour - ask bookie" , "With success more likely than failure - check with bookie" .)