Mashed potato father had just like that (2,3,4,2,1,3)
I believe the answer is:
at the drop of a hat
'just like that 234213' is the definition.
I know nothing about this answer so I can't judge whether it can be defined by this definition.
'mashed potato father had' is the wordplay.
'mashed' indicates anagramming the letters.
'potato'+'father'+'had'='potatofatherhad'
'potatofatherhad' is an anagram of 'AT THE DROP OF A HAT'.
Can you help me to learn more?
(Other definitions for at the drop of a hat that I've seen before include "just like that" , "in no time at all" , "suddenly?" , "without delay" , "When a tile falls to the ground" .)