Wrongly presume to be the greatest (7)
I believe the answer is:
supreme
'the greatest' is the definition.
(I've seen this before)
'wrongly presume' is the wordplay.
'wrongly' indicates anagramming the letters (I've seen this in other clues).
'presume' is an anagram of 'SUPREME'.
'to be' acts as a link.
(Other definitions for supreme that I've seen before include "over the top" , "Greatest in status, authority or power" , "Most excellent" , "Chief" , "Paramount" .)