It’s hard to pigeonhole diverse categories ultimately missing out (9)
I believe the answer is:
esoterica
'out' is the definition.
Although both the answer and definition are singular nouns, I don't understand how they can define each other.
'it's hard to pigeonhole diverse categories ultimately missing' is the wordplay.
'it's hard' indicates anagramming the letters.
'ultimately' suggests the final letters.
'missing' becomes 'rica' (I am not sure about this - if you are sure you should believe this answer much more).
The final letters of 'pigeonhole diverse categories' are 'ees'.
'to'+'ees'='toees'
'toees' is an anagram of 'esote'.
'esote'+'rica'='ESOTERICA'
Can you help me to learn more?
(Other definitions for esoterica that I've seen before include "specialist publications" , "Highly specialised subjects" , "Specialized publications" , "Arcane items" , "Obscure ideas" .)