Solemn imp is odd character in nursery rhyme (6,5)
I believe the answer is:
simple simon
'character in nursery rhyme' is the definition.
The definition suggests a singular noun which matches the answer.
'solemn imp is odd' is the wordplay.
'odd' indicates anagramming the letters.
'solemn'+'imp'+'is'='solemnimpis'
'solemnimpis' anagrammed gives 'SIMPLE SIMON'.
Can you help me to learn more?
(Other definitions for simple simon that I've seen before include "Penniless pie seeker" , "One who met a 14 across" , "idiot" , "I went to the fair" , "Nursery rhyme character" .)