Square up over drinker's last bottle (5)
I believe the answer is:
nerve
'bottle' is the definition.
(both can mean courage)
'square up over drinker's last' is the wordplay.
'square' becomes 'even' (both mean 'owing nothing' as in 'we're all square').
'up' is a reversal indicator (in down clue: letters go upwards).
'over' is an insertion indicator.
'drinker's last' becomes 'r' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'even' in reverse letter order is 'neve'.
'neve' going around 'r' is 'NERVE'.
Can you help me to learn more?
(Other definitions for nerve that I've seen before include "that may be attached to optic?" , "Steel - bottle - neck" , "Fibre carrying impulses from brain to body" , "Hold it and maintain courage" , "Courage to carry on" .)