Aims to get across carrying one (6)
I believe the answer is:
trains
'aims' is the definition.
(as in training a gun on something)
'across carrying one' is the wordplay.
'across' becomes 'trans' (prefix meaning across or over).
'carrying' means one lot of letters goes inside another.
'one' becomes 'i' (Roman numeral).
'trans' placed around 'i' is 'TRAINS'.
'to get' acts as a link.
(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" .)