Give a lecture on obsolete monetary unit (7)

I believe the answer is:

expound

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'lecture on obsolete monetary unit' is the definition.
'expound' can be an answer for 'lecture' (thesaurus). I am unsure of the 'on obsolete monetary unit' bit.

I can't explain the rest of the clue.

Can you help me to learn more?

(Other definitions for expound that I've seen before include "Elaborate" , "Elucidate" , "Put forward detailed argument" , "Explain and interpret" , "Explain meaning in detail" .)

I've seen this clue in The Irish Independent.

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