Old money only limited by education (5)

I believe the answer is:

ducat

I'm a little stuck... Click here to teach me more about this clue!

'old money' is the definition.
(historical type of coin)

'limited by education' is the wordplay.
'limited by' indicates a hidden word (the clue contains or limits the answer).
'DUCAT' is hidden within 'education'.

'only' is the link.
I am not very happy about this link. It may belong to another bit of the clue.

Can you help me to learn more?

(Other definitions for ducat that I've seen before include "tender once" , "Formerly a gold coin of several European countries" , "Mad; fruit" , "Coin once used" , "Old European gold or silver coin" .)

I've seen this clue in the Sydney Morning Herald.

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