Bounding main (5)
I believe the answer is:
ocean
'bounding main' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for ocean that I've seen before include "One of the Seven Seas" , "Large expanse of water" , "Large body of salt water" , "Immense expanse or quantity" , "Large area of water" .)