Sailor absorbed by issue — one can't sink lower than that! (6)
I believe the answer is:
seabed
'one can't sink lower than that' is the definition.
(the very bottom of the sea)
'sailor absorbed by issue' is the wordplay.
'sailor' becomes 'ab' (abbreviation for able seaman).
'absorbed by' is an insertion indicator.
'issue' becomes 'seed' (both can mean one's offspring).
'ab' put within 'seed' is 'SEABED'.
(Other definitions for seabed that I've seen before include "The ocean floor, not for sleeping" , "Debase (anag)" , "Davy Jones's locker" , "marine base" , "Bottom of the ocean" .)