Potentially poisonous creature dispenses with a fox (3)
I believe the answer is:
tod
'fox' is the definition.
(historic term)
'potentially poisonous creature dispenses with a' is the wordplay.
'potentially poisonous creature' becomes 'toad' (some toads are poisonous).
'dispenses with' is a deletion indicator.
'toad' with 'a' removed is 'TOD'.
(Other definitions for tod that I've seen before include "Former English weight for wool - dot it" , "One's alone on this" , "Sly person" , "If you're on it, you're on your own" , "Fox" .)