Guileless young guys missing in class (5)
I believe the answer is:
genus
'class' is the definition.
(I've seen this before)
'guileless young guys missing in' is the wordplay.
'guileless young guys' becomes 'ingenus' (ingenu can mean an innocent or naive young man).
'missing' suggests deleting specific letters.
'ingenus' with 'in' removed is 'GENUS'.
(Other definitions for genus that I've seen before include "species grouped together" , "Taxonomic class between family and species" , "Class of animal" , "Category above species (Biology)" , "Type" .)