Finally bet on neither odd number (8)
I believe the answer is:
thirteen
'number' is the definition.
(thirteen is a kind of number)
'finally bet on neither odd' is the wordplay.
'finally' indicates one should take the final letters.
'on' means one lot of letters goes inside another.
'odd' is an anagram indicator.
The final letter of 'bet' is 't'.
'neither' with letters rearranged gives 'thireen'.
't' placed inside 'thireen' is 'THIRTEEN'.
(Other definitions for thirteen that I've seen before include "Some consider it unlucky" , "Where situation is" , "Supper attendees" , "Number at Last Supper" , "Half of what's left [26D]" .)