Mature and elaborate plan (6)
I believe the answer is:
agenda
'plan' is the definition.
(I know that agenda is a type of plan)
'mature and elaborate' is the wordplay.
'mature' becomes 'age' (synonyms).
'elaborate' is an anagram indicator (an elaborate version of the letters).
'and' with letters rearranged gives 'nda'.
'age'+'nda'='AGENDA'
(Other definitions for agenda that I've seen before include "Topic list for meeting" , "Menu" , "Programme of a meeting" , "Timetable" , "List of items to be dealt with" .)