Being resigned to doing without a mushroom (9)
I believe the answer is:
'being resigned' is the definition.
'doing without a mushroom' is the wordplay.
'doing' becomes 'acting' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'without' indicates putting letters inside ('without' can be similar in meaning to 'outside').
'a mushroom' becomes 'cep' (name for Boletus edulis - an edible mushroom).
'acting' going around 'cep' is 'ACCEPTING'.
'to' acts as a link.
Can you help me to learn more?
(Another definition for accepting that I've seen is " Tolerant".)