Person grinding corn with current on, an exceptional type (3,2,1,7)
I believe the answer is:
one in a million
'an exceptional type' is the definition.
(someone with exceptional qualities)
'person grinding corn with current on' is the wordplay.
'person grinding corn' becomes 'one in a mill' (someone working in a mill).
'with' is a charade indicator (letters next to each other).
'current' becomes 'I' (symbol for electric current).
'oneinamill'+'i'+'on'='ONE IN A MILLION'
(Other definitions for one in a million that I've seen before include "Wonderful person, almost unique?" , "Quite exceptional person" , "someone very special? - with 24a" , "Rare or exceptional person or thing" , "Gem; exceptional person" .)