Machine-gun placed here in comfortable retreat (4)
I believe the answer is:
nest
I believe this clue is a double definition.
'machine-gun placed here' is the first definition.
(a nest is the area where a machine gun is set up)
'comfortable retreat' is the second definition.
'in' is the link.
(Other definitions for nest that I've seen before include "Snuggle in like bird" , "Sent to avian home" , "Avian structure" , "In which to lay eggs or give birth" , "Bird's retreat" .)