Immediately when Derby, say, fell (2,3,4,2,1,3)
I believe the answer is:
at the drop of a hat
'immediately' is the definition.
(I've seen this in another clue)
'when derby say fell' is the wordplay.
I cannot really see how this works, but
'when' could be 'e' (I've seen this in other clues) and 'e' is found in the answer.
'derby' could be 'hat' (I've seen this in another clue) and 'hat' is found in the answer.
This may be the basis of the clue (or it may be nonsense).
(Other definitions for at the drop of a hat that I've seen before include "just like that" , "sudden" , "suddenly?" , "Derby's relegation signalled thus" , "without delay" .)