Content of bag extremely mature (3)
I believe the answer is:
age
'mature' is the definition.
(synonyms)
'content of bag extremely' is the wordplay.
'content of' says the answer is hidden in the clue.
'AGE' can be found hidden inside 'bag extremely'.
(Other definitions for age that I've seen before include "How long something has existed" , "Epoch" , "Years lived so far" , "Historical period" , "Period of time" .)