Told to buy one's boy a big buffalo (5)

I believe the answer is:

bison

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

'a big buffalo' is the definition.
'bison' can be an answer for 'buffalo' (I've seen this before). I'm not sure about the 'a big' bit.

'told to buy one's boy' is the wordplay.
'told' indicates a 'sounds like' (homophone) clue.
'one's boy' becomes 'son' (I've seen this in another clue).
'buy' is a homophone of 'bi'.
'bi'+'son'='BISON'

Can you help me to learn more?

(Other definitions for bison that I've seen before include "O bins could be full of buffalo" , "Buffalo (US)" , "Wild ox of the prairies" , "US prairie cattle" , "Large humped bovid of N America" .)

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

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