Aims to get across carrying one (6)
I believe the answer is:
trains
'aims' is the definition.
(as in training a gun on something)
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for trains that I've seen before include "Pieces of cloth trailing from the backs of gowns" , "They're choo-choos to children" , "Exercises" , "Locomotive-drawn rail cars" , "Prepares for competition" .)