A lot of water a must as man's eating sandwiches (6,3)
I believe the answer is:
tasman sea
'a lot of water' is the definition.
'a must as man's eating sandwiches' is the wordplay.
'sandwiches' says the answer is hidden in the clue (the solution is sandwiched in by the clue).
'TASMAN SEA' is hidden in the letters of 'must as mans eating'.
(Other definitions for tasman sea that I've seen before include "Body of water separating Australia and New Zealand" , "an arm of the Pacific" , "Place to sail far away" , "large quantity of water" , "part of Pacific" .)