Business in small money (4)

I believe the answer is:

coin

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

'small money' is the definition.
'coin' can be an answer for 'money' (coin is a type of money). I am not sure about the 'small' bit.

'business in' is the wordplay.
'business' becomes 'co' (short for company).
'co'+'in'='COIN'

Can you help me to learn more?

(Other definitions for coin that I've seen before include "Metal money disc" , "Money; devise (a word)" , "Invent (a new phrase)" , "Metal currency" , "Penny, say?" .)

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

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