Old theologian followed by boy most likely to succeed (4-2)
I believe the answer is:
odds-on
'most likely to succeed' is the definition.
(odds-on means more likely to happen than not)
'old theologian followed by boy' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'theologian' becomes 'DD' (Doctor of Divinity degree).
'followed by' says to put letters next to each other.
'boy' becomes 'son' (synonyms).
'o'+'dd'+'son'='ODDS-ON'
(Other definitions for odds-on that I've seen before include "With success more likely than favour - ask bookie" , "With success more likely than failure - check with bookie" , "Better than evens" , "More likely to win than lose" , "With success more likely than favour in bookie's phrase" .)