Atlantic ___ (5)
(Other definitions for mid that I've seen before include "Denote the centre point" , "Dim sort of prefix" , "Prefix meaning centre" , "Of the central part" .)
(Other definitions for ocean that I've seen before include "Quantity of water" , "Large body of salt water" , "Large expanse of sea" , "Immense expanse or quantity" , "Great sea" .)