Comrades, trio, cut down, cornered by gun (8)
I believe the answer is:
brethren
'comrades' is the definition.
('brother' can be a synonym of 'comrade')
'trio cut down cornered by gun' is the wordplay.
'trio' becomes 'three' (synonyms).
'cut down' means to remove the last letter.
'cornered by' means one lot of letters goes inside another (I've seen this in other clues).
'gun' becomes 'bren' (Bren is an example).
'three' with its last letter taken away is 'thre'.
'thre' going into 'bren' is 'BRETHREN'.
(Other definitions for brethren that I've seen before include "like John and Charles Wesley, perhaps" , "Fellow-members of church" , "monks" , "Fellow male members" , "Colleagues (in sect)" .)