Simplest mistake; the letters were put in the wrong place (8)
I believe the answer is:
'letters were put in the wrong place' is the definition.
I can't tell whether this defines the answer.
'simplest mistake' is the wordplay.
'mistake' indicates an anagram.
'simplest' with letters rearranged gives 'MISSPELT'.
'the' acts as a link.
Can you help me to learn more?
(Other definitions for misspelt that I've seen before include "badly written" , "Used the wrong letters" , "having put letters in wrong order" , "like velvit?" , "put down in error" .)