Notice superior army officer being mature (5)
I believe the answer is:
adult
'being mature' is the definition.
('adult' can be similar in meaning to 'mature')
'notice superior army officer' is the wordplay.
'notice' becomes 'ad'.
'superior' becomes 'u' (I've seen this in other clues).
'army officer' becomes 'lt' (I've seen this in other clues).
'ad'+'u'+'lt'='ADULT'
(Other definitions for adult that I've seen before include "No child" , "Person of mature age" , "(Of an animal) fully developed" , "Supposedly "responsible" person" , "Full-grown" .)