Good first idea? (4)
I believe the answer is:
gist
'idea?' is the definition.
(thesaurus)
'good first' is the wordplay.
'good' becomes 'g' (abbreviation).
'first' becomes 'ist' (**).
'g'+'ist'='GIST'
(Other definitions for gist that I've seen before include "The pitch on the matter" , "Essential point or general sense of anything" , "Central meaning or theme" , "Significance" , "Core meaning" .)