When Panama falls for no good reason (2,3,4,2,1,3)

I believe the answer is:
at the drop of a hat
'good reason' is the definition.
I know nothing about this answer so I can't judge whether it can be defined by this definition.
'when panama falls for no' is the wordplay.
I cannot quite understand how this works, but
'panama' could be 'hat' (I've seen this in another clue) and 'hat' is found within the answer.
'no' could be 'o' (O resembles 0) and 'o' is present in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for at the drop of a hat that I've seen before include "When a tile falls to the ground" , "When a tile descends" , "without good reason?" , "just like that" , "sudden" .)
