Getting too immersed in it can be fatal (5)
I believe the answer is:
water
'getting' is the definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for water that I've seen before include "Diamond quality" , "Commonplace drink" , "Adam's ale" , "non-alcoholic drink" , "Common liquid" .)