Coin you don't see because you're short-sighted? (8)
I believe the answer is:
farthing
'coin' is the definition.
(farthing is a kind of coin)
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for farthing that I've seen before include "once of minimal value" , "Smallest coin once" , "Old piece" , "of negligible value" , "Obsolete small coin" .)