Such a change may be on the gambler's behalf (3,3,6)

I believe the answer is:

for the better

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'such a change' is the definition.
I know nothing about this answer so I can't judge whether this works.

'be on the gambler's behalf' is the wordplay.
I cannot quite see how this works, but
'be' is within the answer.
an anagram of 'the' is 'het' which is present in the remaining letters.
This explanation may well be incorrect...

'may' acts as a link.

Can you help me to learn more?

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

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