Ship's capacity about the middle of the ocean (7)
I believe the answer is:
galleon
'ship's' is the definition.
(type of ship)
'capacity about the middle of the ocean' is the wordplay.
'capacity' becomes 'gallon' (I've seen this before).
'about' means one lot of letters goes inside another.
'the middle of the ocean' becomes 'e' (I can't explain this - if you can you should believe this answer much more).
'gallon' enclosing 'e' is 'GALLEON'.
Can you help me to learn more?
(Other definitions for galleon that I've seen before include "One might sink" , "Historical warship with three or more masts" , "Ship of the line" , "Trader" , "Large old square-rigged sailing ship" .)