Deprive of weapons (6)
(Other definitions for disarm that I've seen before include "Mollify or remove weapon from" , "Deprive of weapons or win over" , "Demilitarise" , "Defuse" , "Pacify" .)
'deprive of weapons' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for unarm that I've seen before include "Disable" , "Deprive of a weapon" , "Render harmless" , "Remove firing pieces" , "Deactivate" .)