Contradicted requirement to conceal firearm (7)
I believe the answer is:
negated
'contradicted' is the definition.
('negate' can be a synonym of 'contradict')
'requirement to conceal firearm' is the wordplay.
'requirement' becomes 'need' (synonyms).
'to conceal' indicates putting letters inside.
'firearm' becomes 'gat' (gat is a kind of firearm).
'need' placed around 'gat' is 'NEGATED'.
(Other definitions for negated that I've seen before include "Discredited" , "Made ineffective" , "Denied" , "Abrogated" , "Nullified" .)