This fires deadly missiles -- that sucks! No, just the opposite! (8)
I believe the answer is:
blowpipe
This clue is a double definition.
'this fires deadly missiles' is the first definition.
(a blowpipe is used to blow darts)
'that sucks no just the opposite' is the second definition.
(a blowpipe blows rather than sucks)
(Other definitions for blowpipe that I've seen before include "Primitive native weapon" , "Poison-dart tube" , "Missile launcher" , "Tube for hurling darts" , "Tubular weapon" .)