Hercule Poirot originally confused after opening of shallow grave (9)
I believe the answer is:
sepulchre
'grave' is the definition.
(I know that sepulchre is a type of grave)
'hercule poirot originally confused after opening of shallow' is the wordplay.
'originally' indicates taking the first letters.
'confused' indicates anagramming the letters.
'after' is a charade indicator (letters next to each other).
'opening of' suggests taking the first letters.
The initial letter of 'poirot' is 'p'.
The initial letter of 'shallow' is 's'.
'hercule'+'p'='herculep'
'herculep' anagrammed gives 'epulchre'.
'epulchre' put after 's' is 'SEPULCHRE'.
(Other definitions for sepulchre that I've seen before include "Resting place" , "Helps cure (anag)" , "An ancient tomb - whited in Gospels" , "Burial place" , "Burial chamber" .)