Aims to carry one across (6)
I believe the answer is:
trains
'aims' is the definition.
(as in training a gun on something)
'to carry one across' is the wordplay.
'to carry' is an insertion indicator.
'one' becomes 'i' (Roman numeral).
'across' becomes 'trans' (prefix meaning across or over).
'i' going inside 'trans' is 'TRAINS'.
(Other definitions for trains that I've seen before include "They're drawn by locomotive" , "Sets of railway cars" , "Pieces of cloth trailing from the backs of gowns" , "follows exercise regime" , "They're choo-choos to children" .)