Great parents first to know a potential lifesaver (3,4)
I believe the answer is:
gas mask
'a potential lifesaver' is the definition.
The definition suggests a singular noun which matches the answer.
'great parents first to know' is the wordplay.
'great' becomes 'gas' (I am not sure about this - if you are sure you should believe this answer much more).
'parents' becomes 'mas' (ma is a kind of parent).
'first to' indicates taking the first letters.
The initial letter of 'know' is 'k'.
'gas'+'mas'+'k'='GAS-MASK'
Can you help me to learn more?
(Other definitions for gas mask that I've seen before include "It protects against poisonous fumes" , "Wartime breathing aid" , "Wartime safety device" .)