Small volume of trade for a quick sum (5,8)

I believe the answer is:

ready reckoner

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'small volume of trade' is the definition.
I can't judge whether this defines the answer.

'quick sum' is the wordplay.
'quick' becomes 'ready' (similar in meaning).
'sum' becomes 'reckoner' (I can't justify this - if you can you should give a lot more credence to this answer).
'ready'+'reckoner'='READY RECKONER'

'for a' acts as a link.

Can you help me to learn more?

(Other definitions for ready reckoner that I've seen before include "I've done your sums for you" , "Calculation aid" , "Printed tables used in standard computations" , "Calculating aid" , "Book of calculation aids" .)

I've seen this clue in The Globe and Mail.

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