Mashed potato father had right away (2,3,4,2,1,3)
I believe the answer is:
at the drop of a hat
'right away' is the definition.
(I've seen this in another clue)
'mashed potato father had' is the wordplay.
'mashed' is an anagram indicator.
'potato'+'father'+'had'='potatofatherhad'
'potatofatherhad' anagrammed gives 'AT THE DROP OF A HAT'.
(Other definitions for at the drop of a hat that I've seen before include "instantly" , "When pork pie downed" , "in no time at all" , "without delay" , "When a tile descends" .)