Opening fire at the rear, holding up gun (4)
I believe the answer is:
gate
'opening' is the definition.
(a gate is an opening or entrance)
'fire at the rear holding up gun' is the wordplay.
'at the rear' indicates one should take the final letters.
'holding up' means one lot of letters go next to another (some letters go underneath or hold up others).
'gun' becomes 'gat' (US informal term for a gun).
The final letter of 'fire' is 'e'.
'e' after 'gat' is 'GATE'.
(Other definitions for gate that I've seen before include "Attendance figure" , "Punish student" , "Number of people admitted to a sporting event" , "Attendance; entrance" , "Confine to college" .)