Is written in a document in error (5)
I believe the answer is:
amiss
'in error' is the definition.
(I've seen this in another clue)
'is written in a document' is the wordplay.
'written in' means one lot of letters goes inside another.
'document' becomes 'ms'.
'a'+'ms'='ams'
'is' put within 'ams' is 'AMISS'.
(Other definitions for amiss that I've seen before include "Faulty" , "Mistakenly" , "Wrongly" , "Wrorg" , "error" .)