Hard to understand inclusion of prison in area ultimately lacking (6)
I believe the answer is:
arcane
'hard to understand' is the definition.
(I've seen this in another clue)
'prison in area ultimately lacking' is the wordplay.
'prison' becomes 'can' (the can is American slang for prison).
'in' is an insertion indicator.
'ultimately lacking' means to remove the last letter.
'area' with its final letter taken off is 'are'.
'can' going inside 'are' is 'ARCANE'.
'inclusion of' is the link.
This may not be right. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for arcane that I've seen before include "Mysterious - an acre (anag)" , "Recondite" , "Requiring special knowledge" , "Abstruse, mysterious" , "Mysterious, secret or obscure" .)