He was a Dutch painter? Utter nonsense (5)
I believe the answer is:
bosch
'he was a dutch painter? utter nonsense' is the definition.
The definition suggests a singular noun which matches the answer.
'he was a dutch' is the wordplay.
'he' becomes 'Bo'.
'was a dutch' becomes 'sch' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'bo'+'sch'='BOSCH'
Can you help me to learn more?
(Other definitions for bosch that I've seen before include "Hieronymus --, Flemish painter" , "Hieronymous -----, Dutch painter" , "Fantastic Dutch artist" , "Dutch painter known especially for "The Garden of Earthly Delights"" , "Dutch painter of The Garden of Earthly Delights, d. 1516" .)