Nowadays a superior officer is mature (5)
I believe the answer is:
adult
'is mature' is the definition.
('adult' can be similar in meaning to 'mature')
'nowadays a superior officer' is the wordplay.
'nowadays' becomes 'ad'.
'a superior' becomes 'u' (I've seen this in other clues).
'officer' becomes 'lt' (I've seen this in other clues).
'ad'+'u'+'lt'='ADULT'
(Other definitions for adult that I've seen before include "Supposedly "responsible" person" , "X-rated?" , "Older person" , "I wasn't born yesterday" , "(Of an animal) fully developed" .)