Gunmen grabbed guards in charge of arms (8)
I believe the answer is:
heraldic
'of arms' is the definition.
'gunmen grabbed guards in charge' is the wordplay.
'gunmen' becomes 'ra'.
'grabbed' becomes 'held'.
'guards' is an insertion indicator.
'in charge' becomes 'ic'.
'ra' placed within 'held' is 'herald'.
'herald'+'ic'='HERALDIC'
(Other definitions for heraldic that I've seen before include "Relating to coats of arms etc" , "Of armorial bearings" , "related to genealogical study" , "of the coat of arms" , "Of family crests" .)