Rethink of an unfulfilled order? (4,7,5,6)

I believe the answer is:

have another thing coming

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

'rethink of an unfulfilled' is the definition.
I can't tell whether this defines the answer.

'order?' is the wordplay.
I cannot quite see how this works, but
'order?' could be 'om' (Order of Merit) and 'om' is located in the answer.
This may be the basis of the clue (or it may be nonsense).

Can you help me to learn more?

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

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