Sailor drilling into stone floor of ocean (6)
I believe the answer is:
seabed
'floor of ocean' is the definition.
'sailor drilling into stone' is the wordplay.
'sailor' becomes 'ab' (abbreviation for able seaman).
'drilling into' indicates putting letters inside.
'stone' becomes 'seed' (eg the stone from a fruit).
'ab' put into 'seed' is 'SEABED'.
(Other definitions for seabed that I've seen before include "Bottom of the ocean" , "Debase (anag)" , "marine base" , "The ocean floor, not for sleeping" , "Davy Jones's locker" .)