Gunmen manage to find scene of cut-throat activity (3,4)
I believe the answer is:
rat race
'activity' is the definition.
(rat race is a kind of activity)
'gunmen manage to find scene of cut-throat' is the wordplay.
I cannot really see how this works, but
'gunmen' could be 'ra' (abbreviation for Royal Artillery) and 'ra' is found in the answer.
'find' could be 'trace' (I've seen this before) and 'trace' is located in the answer.
No letters remain.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for rat race that I've seen before include "Working life's struggle" , "competitive lifestyle" , "Competition for power" , "nine-to-five" , "Grim struggle" .)