Perhaps Mark once confused grocer with funny rice (7,8)

I believe the answer is:

foreign currency

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'perhaps mark' is the definition.
The definition suggests a singular noun which matches the answer.

'confused grocer with funny rice' is the wordplay.
'confused' indicates an anagram.
'with' means one lot of letters go next to another.
'funny'+'rice'='funnyrice'
'grocer' after 'funnyrice' is 'funnyricegrocer'.
'funnyricegrocer' with letters rearranged gives 'FOREIGN CURRENCY'.

'once' acts as a link.

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