What a tightrope-walker should eat? (8,4)

I believe the answer is:

balanced diet

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'tightrope-walker should eat?' is the definition.
I can't tell whether this defines the answer.

'what a' is the wordplay.
I cannot quite understand how this works, but
'a' could be 'an' and 'an' is found within the answer.
This may be the basis of the clue (or it may be nonsense).

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I've seen this clue in The Daily Express.

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