Some splendid earnest creative thought (4)
I believe the answer is:
idea
'creative thought' is the definition.
(thesaurus)
'some splendid earnest' is the wordplay.
'some' indicates a hidden word (some of the parts of the word are used).
'IDEA' can be found hidden inside 'splendid earnest'.
(Other definitions for idea that I've seen before include "a wheeze" , "Thought or notion (4)" , "Content of cognition" , "Plan" , "Plato's Form is" .)