Mature blue (5)
I believe the answer is:
adult
I believe this clue is a double definition.
'mature' is the first definition.
('adult' can be similar in meaning to 'mature')
'blue' is the second definition.
(I've seen this before)
(Other definitions for adult that I've seen before include "one no longer in minority" , "Person of mature age" , "Mature - grown-up" , "Indecent" , "Older person" .)