Cold marine animal loses tail in salt water (4,3)
I believe the answer is:
dead sea
'water' is the definition.
(I know that Dead Sea is a water)
'cold marine animal loses tail in salt' is the wordplay.
I cannot quite see how this works, but
'cold' could be 'dead' (similar in meaning) and 'dead' is found within the answer.
'marine' could be 'sea' (both can mean 'of the sea') and 'sea' is found within the answer.
This accounts for all the letters.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for dead sea that I've seen before include "Hypersaline lake" , "Middle East saltwater lake, almost 1,300 feet below sea level" , "'Body of water, sounds like it's passed on (4,3)'" , "World's lowest lake" , "Lake in Jordan valley where Scrolls were found" .)