Poirot mistaken about most of folk in case (9)
I believe the answer is:
portfolio
'case' is the definition.
(I know that portfolio is a type of case)
'poirot mistaken about most of folk' is the wordplay.
'mistaken' indicates anagramming the letters.
'about' means one lot of letters goes inside another.
'most of' means to remove the last letter.
'poirot' is an anagram of 'portio'.
'folk' with its last letter taken away is 'fol'.
'portio' going around 'fol' is 'PORTFOLIO'.
'in' is the link.
(Other definitions for portfolio that I've seen before include "responsibility of minister" , "mixed dish" , "A set of investment" , "Case for loose paper, drawings etc" , "Shares" .)