Decisive moment, when to leave a gratuity (7,5)
I believe the answer is:
tipping point
'decisive moment' is the definition.
'tipping point' can be an answer for 'moment' (I have seen 'Critical moment' mean 'tipping point' so perhaps 'moment' could also mean 'tipping point'). I'm unsure of the 'decisive' bit.
'to leave a gratuity' is the wordplay.
I cannot really see how this works, but
'gratuity' could be 'tip' (as in a gratuity to a waiter) and 'tip' is found within the answer.
This may be the basis of the clue (or it may be nonsense).
'when' is the link.
Can you help me to learn more?
(Other definitions for tipping point that I've seen before include "Critical moment" , "when taxi-driver proffers change?" .)