Harmonious like child with a set of books (8)
I believe the answer is:
assonant
'harmonious' is the definition.
(thesaurus)
'like child with a set of books' is the wordplay.
'like' becomes 'as'.
'child' becomes 'son' (son is a kind of child**).
'with' is a charade indicator (letters next to each other).
'a set of books' becomes 'ant'.
'as'+'son'+'ant'='ASSONANT'
(Other definitions for assonant that I've seen before include "having correspondence" , "Having resemblance in sound" , "describing courage with carriage" , "Having similar sounds" , "Like mauve and taupe" .)