Pie portions mixed up, for instance (11)
I believe the answer is:
preposition
'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" .)