Spell out fully the thinking behind old money (7)

I believe the answer is:

expound

'spell out fully the thinking behind' is the definition.

'old money' is the wordplay.
'old' becomes 'ex'.
'money' becomes 'pound' (I've seen this before).
'ex'+'pound'='EXPOUND'

(Other definitions for expound that I've seen before include "Set forth, explain in detail" , "Elucidate" , "Spell out" , "Set out in a detailed and learned way" , "Elaborate" .)

I've seen this clue in The Guardian.

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