Mark has two keys, though they're in short supply (6)
I believe the answer is:
scarce
'in short supply' is the definition.
(both can mean rare)
'mark has two keys' is the wordplay.
'two keys' means two substitutions for 'key'.
'mark has' becomes 'scar' (I can't justify this - if you can you should give a lot more credence to this answer).
'key' becomes 'c' (I've seen this in other clues).
'key' becomes 'e' (I've seen this in other clues).
'scar'+'c'+'e'='SCARCE'
'though they're' is the link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for scarce that I've seen before include "Not enough to satisfy demand" , "Like gold dust" , "Rare shock Conservative getting in" , "Common? Definitely not" , "Insufficient to meet demand" .)