Facts are not first class (5)
I believe the answer is:
genre
'class' is the definition.
(genre can mean a class or type)
'facts are not first' is the wordplay.
'facts' becomes 'gen'.
'not' suggests deleting specific letters.
'first' becomes 'a' (first letter of the alphabet).
'are' with 'a' removed is 're'.
'gen'+'re'='GENRE'
(Other definitions for genre that I've seen before include "Style/category of art" , "Type or style, of art" , "Literary type, species" , "Literary or artistic type or style" , "Kind" .)