One day last month, became mature (5)
I believe the answer is:
adult
'mature' is the definition.
('adult' can be similar in meaning to 'mature')
'one day last month' is the wordplay.
'one' becomes 'a'.
'day' becomes 'd'.
'last month' becomes 'ult' (I've seen this before).
'a'+'d'+'ult'='ADULT'
'became' is the link.
(Other definitions for adult that I've seen before include "Full-grown" , "not fit for children to see" , "used to describe dirty movies" , "Mature, fully-grown" , "Indecent" .)