Very clever person missing one biology class (5)
I believe the answer is:
genus
'biology class' is the definition.
(I've seen this in another clue)
'very clever person missing one' is the wordplay.
'very clever person' becomes 'genius' (I've seen this before).
'missing' 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 "Kind or sort of" , "Category between family and species" , "Division of family" , "Part of the family" , "Taxonomic grouping" .)