Wrongly accept to be an accident (7)
I believe the answer is:
mistake
'accident' is the definition.
(thesaurus)
'wrongly accept' is the wordplay.
'wrongly' becomes 'mis' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'accept' becomes 'take' (to accept is to take possession of).
'mis'+'take'='MISTAKE'
'to be an' is the link.
Can you help me to learn more?
(Other definitions for mistake that I've seen before include "Oversight" , "Fault" , "16 down" , "Wrong action or judgment" , "Error" .)