Ed writes mystery, the strangest (8)
I believe the answer is:
weirdest
'strangest' is the definition.
('weird' can be a synonym of 'strange')
'ed writes mystery' is the wordplay.
'mystery' is an anagram indicator.
'ed'+'writes'='edwrites'
'edwrites' with letters rearranged gives 'WEIRDEST'.
'the' acts as a link.
(Other definitions for weirdest that I've seen before include "Most unearthly" , "Most bizarre" , "Most uncanny!" , "Most strikingly odd" .)