Clever person dropped one class (5)
I believe the answer is:
genus
'class' is the definition.
(I've seen this before)
'clever person dropped one' is the wordplay.
'clever person' becomes 'genius' (genius can mean a very intelligent person).
'dropped' indicates named letters should be taken away.
'one' becomes 'i' (Roman numeral).
'genius' with 'i' taken out is 'GENUS'.
(Other definitions for genus that I've seen before include "Part of the family" , "group of organisms" , "Latin class?" , "Division of family" , "Category above species (Biology)" .)