Clever person misses one class (5)
I believe the answer is:
genus
'class' is the definition.
(I've seen this before)
'clever person misses one' is the wordplay.
'clever person' becomes 'genius' (genius can mean a very intelligent person).
'misses' indicates named letters should be taken away.
'one' becomes 'i' (Roman numeral).
'genius' with 'i' removed is 'GENUS'.
(Other definitions for genus that I've seen before include "Class of animal" , "group of organisms" , "Latin class?" , "Biological group containing one or more species" , "Taxonomic class between family and species" .)