Betting odds nevertheless shortened (5)
I believe the answer is:
evens
'betting odds' is the definition.
(I've seen this before)
'nevertheless shortened' is the wordplay.
'nevertheless' becomes 'evenso' ('even so' can be a synonym of 'nevertheless').
'shortened' means to remove the last letter.
'evenso' with its last letter removed is 'EVENS'.
(Other definitions for evens that I've seen before include "Likelihood" , "These are odds, strangely" , "Presses" , "Makes equal" , "50-50 odds" .)