Poisonous fungus given by obnoxious person finally kills agent (9)
I believe the answer is:
toadstool
'poisonous fungus' is the definition.
(I've seen this before)
'obnoxious person finally kills agent' is the wordplay.
I cannot quite understand how this works, but
'person' could be 'tool' (tool is a kind of person) and 'tool' is located in the answer.
The remaining letters 'toads' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
'given by' acts as a link.
Can you help me to learn more?
(Other definitions for toadstool that I've seen before include "what's poisonous maybe" , "Poisonous fungus" , "Umbrella-shaped fungus" , "Umbrella-shaped cap on a stalk" , "Support for little people" .)