Mature and complex plan (6)
I believe the answer is:
agenda
'plan' is the definition.
(I know that agenda is a type of plan)
'mature and complex' is the wordplay.
'mature' becomes 'age' (synonyms).
'complex' is an anagram indicator.
'and' anagrammed gives 'nda'.
'age'+'nda'='AGENDA'
(Other definitions for agenda that I've seen before include "To-do list" , "things to discuss" , "List of items to be discussed" , "List of items for discussion by committee" , "List of items to be dealt with" .)