Repealed demand to purchase gun (7)
I believe the answer is:
negated
'repealed' is the definition.
'demand to purchase gun' is the wordplay.
'demand' becomes 'need' ('need' can be a synonym of 'demand').
'to purchase' indicates putting letters inside.
'gun' becomes 'gat' (gat is a kind of gun).
'need' going around 'gat' is 'NEGATED'.
(Other definitions for negated that I've seen before include "Contradicted" , "Made ineffective" , "Abrogated" , "Cancelled or destroyed the effect of something" , "Nullified" .)