Small saying in tavern when it's knocked back (7,2)
I believe the answer is:
bottoms up
'tavern when it's knocked back' is the definition.
I don't know anything about this answer so I can't judge whether this works.
'small saying in' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'saying' becomes 'bottomup' (I can't justify this - if you can you should give a lot more credence to this answer).
'in' is an insertion indicator.
's' going within 'bottomup' is 'BOTTOMS UP'.
Can you help me to learn more?
(Other definitions for bottoms up that I've seen before include "Toast" , "Your good health!" , "Cheers!" , "Down the hatch!" , "down it goes!" .)