Money: a sort of thing that runs short? (6)
I believe the answer is:
specie
'money' is the definition.
(I've seen this before)
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for specie that I've seen before include "Coinage, metal money" , "Coin as opposed to paper money" , "Money as coins" , "Coins collectively" , "Broken pieces for coined money" .)