Load ton on mythological ship (5)
I believe the answer is:
cargo
'load' is the definition.
('cargo' can be a synonym of 'load')
'ton on mythological ship' is the wordplay.
'ton' becomes 'c'.
'on' is a charade indicator (letters next to each other).
'mythological ship' becomes 'argo' (I've seen this before).
'c'+'argo'='CARGO'
(Other definitions for cargo that I've seen before include "Shipment" , "Ship's freight" , "Consignment" , "Goods carried by vessel or vehicle" , "Load carried" .)