Payload initially carried by mythical ship (5)
I believe the answer is:
cargo
'payload' is the definition.
(I know that payload can be written as Cargo)
'initially carried by mythical ship' is the wordplay.
'initially' says to take the initial letters.
'by' says to put letters next to each other.
'mythical ship' becomes 'argo' (I've seen this in another clue).
The initial letter of 'carried' is 'c'.
'c'+'argo'='CARGO'
(Other definitions for cargo that I've seen before include "Consignment" , "Hold contents" , "This taken on board" , "Goods being transported" , "Shipment" .)