.. while those who don't care just don't give one (4)
I believe the answer is:
toss
'while those who don't care just don't give one' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how they can define each other.
This is all the clue.
Can you help me to learn more?
(Other definitions for toss that I've seen before include "Careless not to give this" , "Flip up into the air - coin or salad" , "Raise a bit of money before opening" , "Ship's movement" , "decision-making process" .)