Perish by eating mushrooms, upsetting someone looking on (9)
I believe the answer is:
spectator
'perish by' is the definition.
I can't tell whether this definition defines the answer.
'eating mushrooms upsetting someone looking on' is the wordplay.
'eating' indicates putting letters inside.
'mushrooms' becomes 'ceps' (cep is a type of mushroom).
'upsetting' is a reversal indicator.
'someone looking' becomes 'tar' (I can't justify this - if you can you should give a lot more credence to this answer).
'on' becomes 'to'.
'ceps' reversed gives 'spec'.
'spec'+'tar'='spectar'
'spectar' going around 'to' is 'SPECTATOR'.
Can you help me to learn more?
(Other definitions for spectator that I've seen before include "Onlooker at game" , "He attends matches" , "One watching a game" , "Looker-on" , "Cat poster (anag.)" .)