Erroneous investment in some mines (8)
I believe the answer is:
mistaken
'erroneous' is the definition.
(I've seen this before)
'investment in some mines' is the wordplay.
'investment' becomes 'stake' (stake is a kind of investment).
'in' is an insertion indicator.
'some mines' becomes 'min' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'stake' going within 'min' is 'MISTAKEN'.
Can you help me to learn more?
(Other definitions for mistaken that I've seen before include "Wrong (in judgement)" , "'Wrong, misguided (8)'" , "Erroneous" , "Wrong, incorrect" , "Wrong-headed" .)