Assert leader mistaken about 100 (7)
I believe the answer is:
declare
'assert' is the definition.
(declaring is a kind of asserting)
'leader mistaken about 100' is the wordplay.
'mistaken' is an anagram indicator.
'about' indicates putting letters inside.
'100' becomes 'c' (Roman numeral).
'leader' is an anagram of 'delare'.
'delare' placed around 'c' is 'DECLARE'.
(Other definitions for declare that I've seen before include "Announce 13 closed" , "State emphatically" , "Proclaim" , "Assert" , "Make clearly known" .)