Summarily shot by rifle (7)
I believe the answer is:
briefly
'summarily' is the definition.
(thesaurus)
'shot by rifle' is the wordplay.
'shot' indicates an anagram.
'by'+'rifle'='byrifle'
'byrifle' with letters rearranged gives 'BRIEFLY'.
(Other definitions for briefly that I've seen before include "Concisely" , "In a few words" , "In a nutshell" , "Fleetingly" , "For a short time" .)