Pub's motto, questionably — this? (7,2)
I believe the answer is:
bottoms up
'this?' is the definition.
I know nothing about this answer so I can't judge whether it can be defined by this definition.
'pub's motto questionably' is the wordplay.
'questionably' indicates anagramming the letters (questionably can mean strange).
'pubs'+'motto'='pubsmotto'
'pubsmotto' anagrammed gives 'BOTTOMS UP'.
(Other definitions for bottoms up that I've seen before include "Toast" , "Your good health!" , "Cheers!" , "down it goes!" , "Down the hatch!" .)