True, one mucked about on the way (2,5)

I believe the answer is:

en route

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'on the way' is the definition.
(synonyms)

'true one mucked' is the wordplay.
'true one' can be anagrammed to 'EN ROUTE'.
Though, I am not sure how this is indicated.

'about' acts as a link.

Can you help me to learn more?

(Other definitions for en route that I've seen before include "In transit (French)" , "March" , "Along the way" , "'On the way (2,5)'" , "On the way - in France" .)

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

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