Poirot’s one act that keeps detectives in two minds (9)
I believe the answer is:
undecided
'in two minds' is the definition.
(I've seen this before)
'poirot's one act that keeps detectives' is the wordplay.
'poirot's one' becomes 'un' (I can't justify this - if you can you should believe this answer much more).
'act' becomes 'deed' (deed is a kind of act).
'that keeps' indicates putting letters inside (inserted letters are kept inside).
'detectives' becomes 'CID' (abbreviation for Criminal Investigation Department).
'deed' placed around 'cid' is 'decided'.
'un'+'decided'='UNDECIDED'
Can you help me to learn more?
(Other definitions for undecided that I've seen before include "Dithering" , "Still up in the air?" , "With mind not yet made up" , "Not settled" , "Irresolute" .)