Pub should have vermouth, wouldn't you say? (5)
I believe the answer is:
innit
'wouldn't you say?' is the definition.
I know nothing about this answer so I cannot judge whether it can be defined by this definition.
'pub should have vermouth' is the wordplay.
'pub' becomes 'inn' (an inn is a type of pub).
'should have' means one lot of letters go next to another.
'vermouth' becomes 'it' (short for 'Italian vermouth' as in 'gin and it').
'inn'+'it'='INNIT'
Can you help me to learn more?
(Other definitions for innit that I've seen before include "word of emphasis" , "Mere spoken punctuation" , "that produces stress for some" , "Common question" , "Isn't it (sl.)" .)