Expected Tertius or Quintus, possibly but not Sextus (4-2)
I believe the answer is:
odds-on
'expected' is the definition.
Both the definition and answer are adjectives. Maybe you can see an association between them that I can't see?
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for odds-on that I've seen before include "Better than evens" , "With success more likely than favour in bookie's phrase" , "A better than even chance of success" , "More likely to win than lose" , "With success more likely than favour - ask bookie" .)