Mature and complex schedule (6)
I believe the answer is:
agenda
'schedule' is the definition.
(synonyms)
'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 "Meet to discuss this" , "Scheme" , "Plan for meeting" , "Schedule of items for meeting" , "List of items for discussion at a meeting" .)