'Ass' infers stupid type (4,5)
I believe the answer is:
sans serif
'type' is the definition.
(sans serif is a kind of type)
'ass infers stupid' is the wordplay.
'stupid' indicates anagramming the letters (in a stupid order).
'ass'+'infers'='assinfers'
'assinfers' is an anagram of 'SANS SERIF'.
(Other definitions for sans serif that I've seen before include "Plain typeface" , "Style of font" , "Plain style of type" , "plain type of face" , "Style of typeface" .)