Believes they're used for cargo (5)
I believe the answer is:
holds
'believes' is the definition.
('hold' can be a synonym of 'believe')
'used for cargo' is the wordplay.
I cannot really understand how this works, but
'used' could be 'old' (similar in meaning) and 'old' is present in the answer.
This explanation may well be incorrect...
'they're' is the link.
Can you help me to learn more?
(Other definitions for holds that I've seen before include "Grips" , "Clutches" , "Maintains" , "Grasps" .)