Dealt with cynical changes by mistake (12)
I believe the answer is:
accidentally
'by mistake' is the definition.
(I've seen this before)
'dealt with cynical changes' is the wordplay.
'with' means one lot of letters go next to another.
'changes' indicates anagramming the letters.
'dealt'+'cynical'='dealtcynical'
'dealtcynical' anagrammed gives 'ACCIDENTALLY'.
(Other definitions for accidentally that I've seen before include "Inadvertently, by chance" , "Not purposefully, by mistake" .)