Straightens out partners following initial mate (5)
I believe the answer is:
evens
'straightens out' is the definition.
(as in something evening itself out)
'partners following initial mate' is the wordplay.
'partners' becomes 'ns' (North and South are partners in the game bridge).
'following' is a charade indicator (letters next to each other).
'initial mate' becomes 'Eve' (first woman in the Bible, mate of Adam).
'ns' put after 'eve' is 'EVENS'.
(Other definitions for evens that I've seen before include "Smooths the odds" , "Levels - odds" , "Makes equal" , "Odds that win identical sum as the stake" , "straight bet" .)