Father had potato mashed without delay (2,3,4,2,1,3)
I believe the answer is:
at the drop of a hat
'without delay' is the definition.
(I've seen this in another clue)
'father had potato mashed' is the wordplay.
'mashed' indicates an anagram.
'father'+'had'+'potato'='fatherhadpotato'
'fatherhadpotato' is an anagram of 'AT THE DROP OF A HAT'.
(Other definitions for at the drop of a hat that I've seen before include "instantly" , "When a tile descends" , "sudden" , "without delay" , "just like that" .)