Straightens up poles after last night (5)
I believe the answer is:
evens
'straightens' is the definition.
(I've seen this in another clue)
'up poles after last night' is the wordplay.
'up' shows that the letters should be reversed in order (in down clue: letters go upwards).
'poles' becomes 'SN' (North and South poles).
'after last night' becomes 'eve' (I can't justify this - if you can you should give a lot more credence to this answer).
'sn'+'eve'='sneve'
'sneve' back-to-front is 'EVENS'.
Can you help me to learn more?
(Other definitions for evens that I've seen before include "Kind of bet where odds are one to one" , "50-50 chance" , "square" , "Smooths the odds" , "In betting, winning the amount staked" .)