Good initial main point (4)
I believe the answer is:
gist
'main point' is the definition.
(thesaurus)
'good initial' is the wordplay.
'good' becomes 'g' (abbreviation).
'initial' becomes 'ist' (ist resembles 1st).
'g'+'ist'='GIST'
(Other definitions for gist that I've seen before include "Drift" , "General meaning or main point" , "Essential point or meaning of, say, the matter" , "Pith, essence" , "Main point of the matter" .)