Northerner must have change of heart, don’t you think? (5)
I believe the answer is:
innit
'don't you think?' is the definition.
'northerner must have change of heart' is the wordplay.
'northerner must have' becomes 'inn' (I can't justify this - if you can you should give a lot more credence to this answer).
'change' becomes 'switch' (switching is a kind of changing).
'of heart' means to look at the middle letters (I've seen 'heart' mean this).
The middle letters of 'switch' are 'it'.
'inn'+'it'='INNIT'
Can you help me to learn more?
(Other definitions for innit that I've seen before include "Isn't it (sl.)" , "tag question" , "that produces stress for some" , "word of emphasis" , "Mere spoken punctuation" .)