It's up north that it may be mined (3)
I believe the answer is:
tin
'it may be mined' is the definition.
The definition suggests a singular noun which matches the answer.
'it's up north' is the wordplay.
'it's' becomes 'it' (I can't explain this - if you can you should believe this answer much more).
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'north' becomes 'N' (abbreviation).
'it' written backwards gives 'ti'.
'ti'+'n'='TIN'
'that' is the link.
Can you help me to learn more?
(Other definitions for tin that I've seen before include "Silvery-white metal used in cans" , "Metal in Argentina" , "Silver-white metal, used to make cans" , "Money" , "roofing material" .)