Signaller’s device weighing practically nothing? (4,5)
I believe the answer is:
very light
This clue is a double definition.
'signaller's device' is the first definition.
(Very light is a type of flare)
'weighing practically nothing?' is the second definition.
(something light does not weigh much)
(Other definitions for very light that I've seen before include "Gun-fired flare" , "Pistol flare" , "not at all difficult to carry" , "Warning signal" , "Signaller's item" .)