You'll find them first-rate in the tavern, by and large (2,3,4)
I believe the answer is:
in the main
'by and large' is the definition.
(I've seen this before)
'you'll find them first-rate in the tavern' is the wordplay.
'you'll find' becomes 'th' (I can't explain this - if you can you should believe this answer much more).
'them' becomes 'em' (abbreviation).
'first rate' becomes 'AI' (resembles At, old-fashioned slang for very good).
'in the' indicates putting letters inside.
'tavern' becomes 'inn' (I've seen this before).
'th'+'em'+'ai'='themai'
'themai' put into 'inn' is 'IN THE MAIN'.
Can you help me to learn more?
(Other definitions for in the main that I've seen before include "Where schools are found" , "Almost entirely" , "Generally - at sea?" , "where to find water" , "nautically?" .)