Mistaken in having taken a toadstool for a mushroom? (4,5)

'having taken a toadstool for a mushroom?' is the definition.
I know nothing about this answer so I can't judge whether it can be defined by this definition.

'mistaken' is the wordplay.
I cannot quite understand how this works, but
'mistaken' could be 'wrong' (similar in meaning) and 'wrong' is found in the answer.
The remaining letters 'dead' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).

'in' acts as a link.

Can you help me to learn more?