Very bottom bottomless in lowest lake! (4,3)
I believe the answer is:
dead sea
'lowest lake' is the definition.
(lake below sea level)
'very bottom bottomless' is the wordplay.
'very' becomes 'dead' (dead can mean very or absolutely, eg 'dead wrong').
'bottom' becomes 'seat' (synonyms).
'bottomless' means to remove the last letter (the letter at the bottom is removed).
'seat' with its last letter taken away is 'sea'.
'dead'+'sea'='DEAD SEA'
'in' is the link.
(Other definitions for dead sea that I've seen before include "Hypersaline lake" , "Inland salt lake on Israel-Jordan border" , "River Jordan flows into this" , "R. Jordan lake" , "Inland salt lake in the Jordan valley of Israel" .)