Matures a long time (4)
I believe the answer is:
ages
I believe this clue is a triple definition!
'matures' is the first definition.
('age' can be a synonym of 'mature')
'a long' is the second definition.
(thesaurus)
'time' is the third definition.
(I have seen 'Long time' mean 'ages' so perhaps 'time' could also mean 'ages')
(Other definitions for ages that I've seen before include "Rock of - - (hymn)" , "Epochs" , "Awfully long time" , "Grows old like sage" , "... of Man, according to WB Yeats" .)