Sceptic ate this mistakenly (7)
I believe the answer is:
atheist
'sceptic' is the definition.
(I've seen this before)
'ate this mistakenly' is the wordplay.
'mistakenly' indicates anagramming the letters.
'ate'+'this'='atethis'
'atethis' with letters rearranged gives 'ATHEIST'.
(Other definitions for atheist that I've seen before include "One disbelieving in the existence of God or gods" , "One believes there are no gods" , "I don't believe" , "One denies the existence of a god" , "Person who believes there is no God" .)