Six feet under, tailless marine mammal in lowest lake (4,3)
I believe the answer is:
dead sea
'lowest lake' is the definition.
(lake below sea level)
'six feet under tailless marine mammal' is the wordplay.
'six feet under' becomes 'dead' (six feet under means dead and buried).
'tailless' means to remove the last letter (the tail of the word is removed).
'marine mammal' becomes 'seal' (I've seen this before).
'seal' with its final letter taken off is 'sea'.
'dead'+'sea'='DEAD SEA'
'in' acts as a link.
(Other definitions for dead sea that I've seen before include "R. Jordan lake" , "River Jordan flows into this" , "World's lowest lake" , "Hypersaline lake" , "Lake in Jordan valley where Scrolls were found" .)