Oven ruined before end of film with poisonous substance (5)
I believe the answer is:
venom
'poisonous substance' is the definition.
(I know that venom is a type of animal toxin)
'oven ruined before end of film' is the wordplay.
'ruined' indicates an anagram.
'before' says to put letters next to each other.
'end of' suggests the final letters.
The last letter of 'film' is 'm'.
'oven' with letters rearranged gives 'veno'.
'veno'+'m'='VENOM'
'with' acts as a link.
(Other definitions for venom that I've seen before include "ill will" , "Toxic substance" , "Poison; malice" , "Spite: toxin" , "Virulence" .)