Pie portions mixed up, for instance (11)

I believe the answer is:

preposition

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'instance' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.

'pie portions mixed up' is the wordplay.
'mixed up' indicates anagramming the letters.
'pie'+'portions'='pieportions'
'pieportions' anagrammed gives 'PREPOSITION'.

'for' is the link.

Can you help me to learn more?

(Other definitions for preposition that I've seen before include "Word like after or for (grammar)" , "Possibly in" , "One is in" , ""Before" or "after", say" , "Part of speech" .)

I've seen this clue in The Telegraph.

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