Ocean floor beginning to claim sailor (6)
I believe the answer is:
seabed
'ocean floor' is the definition.
(synonyms)
'beginning to claim sailor' is the wordplay.
'beginning' becomes 'seed' (seed is a kind of beginning).
'to claim' indicates putting letters inside (inserted letters are claimed or taken in).
'sailor' becomes 'ab' (abbreviation for able seaman).
'seed' enclosing 'ab' is 'SEABED'.
(Other definitions for seabed that I've seen before include "Bottom of the ocean" , "marine base" , "Debase (anag)" , "Davy Jones's locker" , "The ocean floor, not for sleeping" .)