Endless poison's fed to keeper (7)
I believe the answer is:
curator
'keeper' is the definition.
(curator is a kind of keeper)
'endless poison's fed to' is the wordplay.
'endless' means to remove the last letter.
'poison' becomes 'curare' (curare is a poisonous plant).
'fed' is an insertion indicator (one word is 'fed' to another).
'curare' with its last letter taken off is 'curar'.
'curar' placed around 'to' is 'CURATOR'.
(Other definitions for curator that I've seen before include "Artistic director" , "Cultural employee" , "Museum keeper" , "Custodian of a collection" , "Museum's administrative head" .)