Maybe monthly entering moon's final stage (5)
I believe the answer is:
imago
'final stage' is the definition.
(I've seen this before)
'maybe monthly entering moon's' is the wordplay.
'maybe monthly' becomes 'mag' (a monthly magazine).
'entering' means one lot of letters goes inside another.
'moon' becomes 'Io' (moon of Jupiter).
'mag' put inside 'io' is 'IMAGO'.
(Other definitions for imago that I've seen before include "For example, monarch ultimately" , "ideal representation" , "latest development in cricket?" , "Adult insect after metamorphosis" , "Beetle, for example" .)