Clever person lost one type (5)
I believe the answer is:
genus
'type' is the definition.
(I've seen this before)
'clever person lost one' is the wordplay.
'clever person' becomes 'genius' (genius can mean a very intelligent person).
'lost' 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 "Taxonomic grouping" , "Latin class?" , "Class of animal" , "Category between family and species" , "Type" .)