What's carried primarily on ship (5)
I believe the answer is:
cargo
'ship' is the definition.
(cargo ship is a kind of ship)
'what's carried primarily on' is the wordplay.
'what's' becomes 'argo' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'primarily' indicates taking the first letters.
'on' is a charade indicator (letters next to each other).
The initial letter of 'carried' is 'c'.
'argo' after 'c' is 'CARGO'.
Can you help me to learn more?
(Other definitions for cargo that I've seen before include "Hold contents" , "Freight on board ship" , "Freight, load" , "Stuff on board" , "Freight of ship, plane or truck" .)