Water now overheating changed at once, saving time (5)
I believe the answer is:
ocean
'water' is the definition.
(oceans are bodies of water)
'changed at once saving time' is the wordplay.
'changed' indicates an anagram.
'saving' is a deletion indicator (saving can mean except).
'time' becomes 't' (abbreviation).
'at'+'once'='atonce'
'atonce' with 't' taken away is 'aonce'.
'aonce' is an anagram of 'OCEAN'.
'now overheating' is the link.
I am not very happy about this link. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for ocean that I've seen before include "Huge body of water" , "lot of liquid" , "Major sea" , "Big sea" , "It's pretty wet!" .)