He knows nothing amusing or out of place (9)
I believe the answer is:
ignoramus
'he knows nothing' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'amusing or out of place' is the wordplay.
'out of place' indicates an anagram.
'amusing'+'or'='amusingor'
'amusingor' anagrammed gives 'IGNORAMUS'.
Can you help me to learn more?
(Other definitions for ignoramus that I've seen before include "One who knows nothing" , "unwitting person" , "Idiot" , "Simpleton" , "one in a state of bliss?" .)