Mature lawyer retired last month (5)
I believe the answer is:
adult
'mature' is the definition.
(similar in meaning)
'lawyer retired last month' is the wordplay.
'lawyer' becomes 'da' (abbreviation for District Attorney).
'retired' says the letters should be written in reverse.
'last month' becomes 'ult' (short for 'last month' in correspondence).
'da' in reverse letter order is 'ad'.
'ad'+'ult'='ADULT'
(Other definitions for adult that I've seen before include "not green" , "Full-grown" , "(Of an animal) fully developed" , "Blue" , "X-rated?" .)