Outline is mostly accurate (6)
I believe the answer is:
precis
'outline' is the definition.
(I know that outline can be written as precis)
'mostly accurate' is the wordplay.
'mostly' means to remove the last letter (most but not all of the word).
'accurate' becomes 'precise' (similar in meaning).
'precise' with its last letter removed is 'PRECIS'.
'is' is the link.
(Other definitions for precis that I've seen before include "Summary, short version" , "Abstract; make summary (of text)" , "Digest - summary" , "Summarize" , "Sketchy summary" .)