Endlessly astonish lifesaver with nitrogen river (6)
I believe the answer is:
amazon
'river' is the definition.
(Amazon is an example)
'endlessly astonish lifesaver with nitrogen' is the wordplay.
'endlessly' means to remove the last letter.
'astonish' becomes 'amaze' (synonyms).
'lifesaver with' becomes 'o' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'nitrogen' becomes 'N' (N is the chemical symbol for nitrogen).
'amaze' with its last letter taken off is 'amaz'.
'amaz'+'o'+'n'='AMAZON'
Can you help me to learn more?
(Other definitions for amazon that I've seen before include "seller on the Internet" , "Great river of S America, or a tough woman" , "woman soldier" , "Strong, aggressive woman" , "Brazilian river" .)